EDP Sciences
Free Access
Issue
A&A
Volume 565, May 2014
Article Number A27
Number of page(s) 62
Section Stellar atmospheres
DOI https://doi.org/10.1051/0004-6361/201322696
Published online 28 April 2014

Online material

Appendix A: Additional tables

Table A.1

Available spectra for WN stars in the LMC.

Table A.2

LMC WN stars analyzed in this work.

Table A.3

Number of ionizing photons and Zanstra temperatures for WN stars in the LMC.

Appendix B: Comments on individual stars

Preliminary remark on mass-loss rates. In our models, clumping is parameterized by means of the “density contrast” D (see Sect. 3). We adopt D = 10 throughout our analyses. Other authors used different assumptions for the degree of clumping, or neglected the wind inhomogeneities. As a consequence of the scaling invariance described by Eq. (2), the impact of the clumping contrast on the empirical mass-loss rates that are derived from recombination lines is simply D− 1/2. Therefore, when comparing mass-loss rates with the results from other authors in the following text, we scale their values to our assumptions of D = 10.

BAT99 1

is classified as WN3b in the BAT99 catalog. It shows no periodic radial velocity variations, but seems to be a runaway star, according to Foellmi et al. (2003b). We find no hints for binarity in the spectra and treat them as single stars. Bonanos et al. (2009) derived a stellar temperature of T = 85 kK on the basis of CMFGEN models from Smith et al. (2002), which is slightly lower than T = 89 kK, as derived in our analysis. The same value has been obtained by HK2000 with a previous version of our code. Willis et al. (2004) have used their FUSE spectra to derive a terminal velocity of v = 2745 km s-1 from the black edge of the P Cygni profiles present in the far UV (FUV), whereas Niedzielski et al. (2004) estimated terminal velocities of v = 1265 − 2506 km s-1 from the P Cygni lines in the IUE-range with the same method. We prefer v = 1600 km s-1 from the width of the optical emission lines in accordance with HK2000. A terminal velocity of v = 2754 km s-1 would result in emission lines which are considerably broader than observed (see Sect. 4.3 for details).

BAT99 2

is one of the two WN2 stars (the other one is BAT99 49) in the LMC, and clearly among the hottest stars of the sample. Actually, BAT99 2 is one of only two WR stars in the LMC able to ionize a He ii-region (Nazé et al. 2003b). These authors conclude from the nebular He ii-flux that the exciting star delivers 4 × 1047 He ii ionizing photons per second. This agrees with our final model, which produces 3.3 × 1047 He ii ionizing photons per second.

A model with a stellar temperature of 90 − 100 kK (Nazé et al. 2003a) cannot reproduce the observed spectra. The synthetic spectra below T = 110 kK show C ivλ 1548 and C ivλ 5801 lines, which are not observed. All appropriate models with stellar temperatures above T = 110 kK fall into the regime of parameter degeneracy of the model grid (see Sect. 4.2). In this case, the determination of the temperature was based on a slightly better fit for the N vλ 1242 and N vλ 4603 lines at T = 141 kK. Between fits with temperatures of 130 kK and 160 kK, the slope of the optical/UV continuum (and thus the inferred reddening parameter) hardly changes, but the luminosity increases from log  (L/L) = 5.35 to 5.58 due to the bolometric correction.

Foellmi et al. (2003b) reported the detection of hydrogen emission in the spectrum of BAT99 2. We cannot exclude a hydrogen mass-fraction of XH ~ 0.1, but the observation is perfectly consistent with zero hydrogen. The presence of hydrogen would be unexpected for a WN star of such high effective temperature and, thus, advanced evolution stage.

Foellmi et al. (2003b) find that the radial velocity of BAT99 2 differs from the mean vrad of their WN sample by about −120 km s-1 and suggest that this star might be a runaway object.

BAT99 3

is listed as spectral type WN4b in the BAT99 catalog. For the first time, stellar and wind parameters for this object are presented here.

BAT99 5

is the second of the two WN2b stars (Foellmi et al. 2003b) in the LMC. BAT99 2 and BAT99 5 exhibit very similar spectra that can be reproduced by the same grid model. The temperature of BAT99 5 was previously determined to be only 71 kK by HK2000, but with today’s line-blanketed models much higher temperatures are adequate, as discussed above for BAT99 2. However, we note that this star is located within the regime of parameter degeneracy (see Sect. 4.2). Massey et al. (2000) suggest an initial mass of Minit> 40 M, while the Geneva tracks in Fig. 10 indicate slightly less than 30 M.

BAT99 5 is suspected to have an OB-companion by Smith et al. (1996), in reference to absorption features visible in the spectrum from Torres-Dodgen & Massey (1988), but the authors mention that these features could also be artifacts from the subtraction of nebular lines. According to Foellmi et al. (2003b), the spectrum shows no radial velocity variations. Like BAT99 2, BAT99 5 was not detected in X-rays by Guerrero & Chu (2008b). As we also find no indications of a companion in the spectrum, we consider this star to be single. If the spectroscopic twin of BAT99 5, namely BAT99 2, is indeed a runaway star as suggested by Foellmi et al. (2003b), the single star evolution of BAT99 5 and the binary evolution of BAT99 2 have led to almost identical products.

BAT99 6

is one of the WN binary systems (period of 2 d) listed in BAT99, but has been demoted since the publication of this catalog. Niemela et al. (2001) reclassified it as O3 f*+O on the basis of its optical spectrum, suggesting that the system contains four stars (two close pairs). On the other hand, Koenigsberger et al. (2003) conclude that the system does not comprise more than two luminous stars. Unfortunately, we do not have optical spectra of this star, so that the stellar parameters are derived from the UV spectrum and the photometry alone. We achieved a reasonable fit, although this binary is fitted as a single star. According to Niemela et al. (2001) the total mass of the system is probably over 80 M.

BAT99 7,

classified as WN4b (BAT99), shows strong emission lines with a round line shape. These line shapes can only be reproduced assuming a high rotational velocity of vrot = 2200 km s-1. Thus, this star is a prototype for the so-called “round line” stars, which are characterized by strong and broad emission lines with round line profiles. The correlation of these line shapes with the stellar rotation has recently been investigated by Shenar et al. (2014). These authors confirm that rotation can account for the spectral characteristics of BAT99 7, but only in connection with a strong magnetic fields that force the wind to co-rotate.

We derive a stellar temperature of T = 158 kK, which is the hottest of all WN4 stars. However, we note that this stars falls into the regime of parameter degeneracy (see Sect. 4.2). The derived stellar temperature is conspicuously higher than values obtained by Koesterke et al. (1991, T = 90 kK, using pure helium models. HK2000 obtained a temperature of T = 100 kK with unblanketed model atmospheres.

BAT99 12

is a transition type O2 If*/WN5 star, according to Crowther & Walborn (2011). Schnurr et al. (2008) argued that the star is most likely a runaway, as already suggested by Massey et al. (2005). Several spectra are at hand for the analysis of this star. In the UV range an HST spectrum and multiple observations with the IUE satellite are available, although only two IUE short-wavelength spectra are in accordance with the HST observation. All IUE long-wavelength spectra exhibit a substantial offset to the rest of the observed SED. Therefore, we have ignored these IUE data. In the optical spectral range, the AAT spectrum and one spectrum observed by Foellmi et al. (2003b) complement each other in wavelength coverage. All spectra can be fitted with the same model, which gives us confidence in the derived stellar parameters.

We derived a stellar temperature of T = 50 kK, confirming the value obtained by Doran & Crowther (2011) and Doran et al. (2013). A lower temperature limit of T> 42 kK was derived by Massey et al. (2005) with the FASTWIND model atmosphere code (Puls et al. 2005). However, at these cooler temperatures neither the N ivλ 4060 nor the N vλλ 4604,4620 lines can be satisfactorily reproduced. In comparison to the study carried out by Doran et al. (2013), we derived the same luminosity, while the mass-loss rate is 0.1 dex lower. In contrast, Massey et al. (2005) derived a mass-loss rate that is 0.23 dex higher. We achieve the best fit with synthetic spectra for a hydrogen mass-fraction of XH = 0.5, which is 0.1 dex lower than previously derived by Doran et al. (2013).

Schnurr et al. (2008) reported radial velocity variations of this star with a period of 3.2 d. For the companion, no spectral features are detected. The SED is well reproduced by a single-star model, thus we expect that the companion does not contribute much to the bolometric luminosity of the binary system.

BAT99 13

is the only WN10 star (BAT99) in the LMC. It has been analyzed before by Crowther et al. (1995b) and Pasquali et al. (1997). The former derived a stellar temperature of T = 29.7 kK with unblanketed atmosphere models. The analysis by Pasquali et al. (1997) with line blanketed models obtained a higher temperature of T = 33 kK. Our best fit is obtained with a model of T = 28 kK. At a temperature of T = 32 kK (one grid step higher) the fit is also reasonable, with the exception of a considerably over-predicted He iiλ 4686 line. Thus, we preferred the model with the lower temperature (T = 28 kK). Since the emission line strength of the He iiλ 4686 line is slightly too low at these temperatures, the “real” temperature is probably marginally higher. But this has only a minor impact on the other stellar parameters. The mass-loss rate derived in our analysis is more than a factor of three below the previous values obtained by Crowther et al. (1995b) and Pasquali et al. (1997).

Bonanos et al. (2009) compared the observed SED with the continua of a 45 kk WN model calculated by Smith et al. (2002) with CMFGEN. From this comparison they found an infrared excess, which cannot be confirmed by our analysis (see Fig. C.4). These different results arise from the high temperature assumed by Bonanos et al. (2009) for this WN10 star. Models calculated with this temperature cannot consistently reproduce the observed spectra.

BAT99 14

is listed as WN+OB? binary candidate without a period in the BAT99 catalog. Foellmi et al. (2003b) reclassified it as WN4o(+OB), but they did not find significant periodical variations in their radial velocity data, concluding that this object is probably not a short-period binary. However, they find absorption lines superimposed on the emission lines, which they attribute to a nearby visual companion. Since our analysis is based on the same spectra, we treat this star as a binary suspect until more appropriate data are available. Stellar and wind parameters are derived in this work for the first time.

BAT99 15

is classified as WN4b (BAT99). It has already been analyzed by Koesterke et al. (1991) and HK2000. We obtained approximately the same stellar temperature as HK2000, but a factor of two lower mass-loss rate. A higher mass-loss rate, however, results in emission lines which are substantially stronger than observed. Furthermore, we derived a slightly higher color excess of Ebv = 0.08 mag, which gives rise to the higher luminosity derived in our analysis.

BAT99 16

was classified as WN7h by Schnurr et al. (2008). We achieve the best fit at a stellar temperature of T = 50 kK, whereas T = 33 kK, T = 34.8 kK and T = 35.5 kK have been derived by Koesterke et al. (1991), Crowther & Smith (1997), and HK2000, respectively. Such a low temperature, however, would completely spoil the fit of the He and N lines. For example, the line ratio of the He iλ 5877 to He iiλ 5412 is much higher than observed in the model with a temperature of 35 kK. We attribute these differences to the unblanketed model atmospheres used by the former authors.

We found a mass-loss rate of log  (/ (M/ yr)) = −4.64. The same value was derived by Crowther & Smith (1997), while Koesterke et al. (1991) obtained a mass-loss rate almost a factor of two lower. HK2000, on the other hand, derived a mass-loss rate nearly a factor of two higher. We estimate a hydrogen mass-fraction of XH = 0.3 from the best fitting models, slightly higher compared to the value derived by HK2000.

BAT99 17

is listed as WN4o in the BAT99 catalog. Bonanos et al. (2009) have analyzed this star, using CMFGEN models by Crowther (2006). They derived an effective temperature of Teff = 52 kK, a luminosity of log  (L/L) = 5.4 and a mass-loss rate of log  (/ (M/ yr)) = −4.85. In our analysis, the best overall fit is achieved with a model corresponding to an effective temperature of Teff = 65 kK. We note that a slightly better fit of the He iiλ 5412 and the He iλ 5877 lines can be achieved at an effective temperature of Teff = 55 kK, although the fit quality of all nitrogen lines and the He iiλ 1641,4201,4339,4542 is reduced compared to the model with Teff = 65 kK. The luminosity derived in this work is factor of two higher than the value obtained by Bonanos et al. (2009), while the mass-loss rate and the terminal velocity are nearly the same. The luminosity increase in comparison to the former study by Bonanos et al. (2009) originates from the higher temperature and thus higher bolometric correction derived in our analysis.

BAT99 18

is a WN3(h) star (Foellmi et al. 2003b), which had never been analyzed before by means of model atmospheres. We confirm the presence of hydrogen (XH = 0.2), i.e., the corresponding classification.

BAT99 19

was classified as WN4b+OB? in the BAT99 catalog. Foellmi et al. (2003b) reported a period of 17.99 d and specified the companion to be an O5: star. In the spectrum from Foellmi et al. (2003b), small absorption lines are superimposed on the emission lines. Due to the weakness of these lines we expect that the contribution of the companion to the bolometric luminosity is only minor. Actually, we achieved a reasonable fit with our single-star model although a high rotational velocity (vrotsini = 2000 km s-1) is necessary to reproduce the round emission lines, as has already been remarked by Breysacher (1981). Thus, this star belongs to the category of the so-called “round line” stars. The correlation of these line shapes with the stellar rotation has recently been investigated by Shenar et al. (2014).

HK2000 have derived a stellar temperature of T = 70.8 kK one grid step lower compared to our new results, which engender a slightly better fit of the UV and optical line spectra at T = 79 kK. We attribute these differences in the models to the line blanketing not incorporated in the models by HK2000. Both results are considerably above the 50 kK obtained by Koesterke et al. (1991) with pure helium models. The mass-loss rate presented in this paper is marginally lower compared to HK2000, if we take the higher terminal velocity v = 2500 km s-1 derived by these authors into account. The SED fit results in nearly the same luminosity and color excess as obtained by HK2000.

BAT99 21

is listed as WN4+OB binary candidate without a period in the BAT99 catalog. Foellmi et al. (2003b) reclassified this object as WN4o(+OB). According to these authors, the radial velocity variations are only marginal, concluding that this object is probably not a short-period binary. They further mentioned that a visual companion situated 2″ away contributes to the observed flux and causes the absorption lines in the spectrum. We analyze the object as a single WN star, although a substantial flux contribution from the companion is expected due to the sizable absorption lines of the companion. More appropriate data is needed to ensure accurate model estimates and to verify the binary status. Until this data is available, we treat this star as a binary suspect.

BAT99 22

is an LBV candidate according to Humphreys & Davidson (1994), Crowther et al. (1995a), and Pasquali et al. (1997), which is listed as WN9h star by BAT99. So far, similar stellar parameters have been reported by Schmutz et al. (1991), Crowther et al. (1995a), and Pasquali et al. (1997). These authors derived stellar temperatures in the range from T = 28.5 kK ... 35.2 kK with unblanketed and blanketed model atmosphere codes, respectively. We obtain a stellar temperature of T = 32 kK, matching the hitherto known temperature range. However, we can exclude the higher as well as the lower temperature, since the He iiλ 4686 line would be considerably overpredicted at 35 kK and underpredicted at 28 kK, respectively.

This star is one of three LMC WN stars that has been detected at 24 μm with the IRAC instrument aboard the Spitzer space telescope (Bonanos et al. 2009). It is known to show a huge infrared excess (Glass 1984; Stahl et al. 1984), which is also visible in our SED fit. According to Allen & Glass (1976), Cowley & Hutchings (1978), and Stahl et al. (1984), a M2 supergiant contributes to the near-infrared flux, and thus probably causes the infrared excess. Schmutz et al. (1991) concluded that the M supergiant is not physically bound to BAT99 22, rather incidentally located along the same line of sight. Observations by Heydari-Malayeri et al. (1997) with ESO NTT SUSI show that the M supergiant is closer than 0.12″, which points to a binary system according to these authors. However, no significant variations have been found in the recent radial-velocity study by Schnurr et al. (2008), which militates against a short-period binary.

A comparison of the spectrum obtained by Schnurr et al. (2008) with that shown in Cowley & Hutchings (1978) indicates that at least one faint TiO band at 5167 Å is visible in the spectrum from Schnurr et al. (2008). Therefore, our results for BAT99 22 are also slightly effected by the late-type supergiant, since our analysis is partially based on this spectrum. Due to the weakness of this feature, however, we consider the uncertainty of the WN parameters introduced by the contribution of the M supergiant to the flux in the optical spectral range as small.

Vink (2007) found intrinsic line depolarization for the He iiλ 6560 line, suggesting an asymmetry in the wind, which is probably either caused by an binary companion or rapid rotation. Weis (2003) found evidence for a nebula associate to BAT99 22, which is not spatially resolved by the available observations. A circumstellar shell has already been proposed by Stahl et al. (1984). These authors concluded that the excess in the L band is too large to be explained by a late supergiant only.

The UV and optical spectra (flux-calibrated) are consistent with a luminosity of log  (L/L) = 5.75. Within the uncertainties, this is equal to the findings of Schmutz et al. (1991) and Crowther et al. (1995a), whereas log  (L/L) = 5.9 was obtained by Pasquali et al. (1997). The best fitting model requires a mass-loss rate of log  (/ (M/ yr)) = −4.85, which is lower but comparable with the previous results (Schmutz et al. 1991; Crowther et al. 1995a; Pasquali et al. 1997).

BAT99 23

was classified as WN3(h) by Foellmi et al. (2003b) due to the hydrogen emission detected by these authors. Our best fit is achieved with a hydrogen-free grid-model, suggesting that the hydrogen mass fraction in the atmosphere of this star is below XH = 0.1. As far as we know, no analysis based on stellar atmosphere models has been published for this object yet.

BAT99 24

is listed as WN4b in the BAT99 catalog. For the first time, we present stellar and wind parameters for this star derived with modern stellar atmospheres. With a stellar temperature of T = 100 kK and a terminal velocity of v = 2400 km s-1, it belongs to the hottest stars in our sample with one of the fastest winds. However, we note that this star is located within the regime of parameter degeneracy (see Sect. 4.2 for details).

BAT99 25

was classified as WN4ha by Foellmi et al. (2003b), rejecting the previous WN3 classification. Since we could not find parameters of this object in the literature for comparison, this object had probably never been analyzed before.

BAT99 26

is a WN4b star (BAT99) with stellar and wind parameters typical for early WN stars. The only exception is the mass-loss rate, which is lower compared to the average of the other WN4b stars in our sample. The parameters derived in our analysis are the first made public for this star.

BAT99 27

has a known B-type companion, although Foellmi et al. (2003b) concluded that this star is probably not a short-periodic binary because no radial velocity variations were detected. Since absorption lines of the companion are also visible in the spectra obtained by these authors, they have slightly reclassified the system to WN5b(+B1 Ia). High resolution observations with the Wide Field Camera 3 (WFC3) aboard the HST by P. Massey (HST Proposal 12940) could not resolve this object in multiple components. Whether the B-type star is physically bound to the WR star or only located in the line of sight is still unclear.

The early B-type companion significantly contributes to the flux of the whole system. Therefore, our analysis of BAT99 27 as a single WN star is considerably affected by the companion. This is the reason that no conclusive fit could be established in this work. The disentanglement of the spectra will be a subject of a forthcoming paper. However, the emission lines have a broad and round line shape, suggesting that the WN component belongs to the so-called “round line” stars. To account for the round line shape, the synthetic spectra is convolved with a rotation profile, corresponding to a rotational velocity of vrot = 1000 km s-1. The correlation of the line shapes with the stellar rotation has recently been investigated by Shenar et al. (2014).

BAT99 29

was revealed as a binary with a period of 2.2 d by Foellmi et al. (2003b) on the basis of their radial velocity studies. These authors classified this object as WN4b+OB. They note a reduced line intensity of the hydrogen lines due to the presence of a companion. However, a reasonable fit of the Balmer lines can also be established with a single-star model (although the Hα line is slightly overpredicted by the model), suggesting that the flux contribution of the OB companion is rather small in this binary system.

BAT99 30

is listed as WN6h in the BAT99 catalog. Stellar temperatures in the range T = 33 ... 39.8 kK have been derived by Koesterke et al. (1991), Crowther & Smith (1997), and HK2000 with unblanketed stellar atmosphere models. We achieved the best fit with a stellar temperature of T = 47 kK. At this temperature, our models reproduce the observed He i/He ii ratio as well as the N iii/N iv ratio (with a trend to slightly higher temperatures), whereas a clear mismatch is obtained with the low temperatures derived in the previous studies. This discrepancy probably originates from the line-blanketing effect included in our models. Similar values for the luminosity and the mass-loss rate have been obtained by Koesterke et al. (1991), Crowther & Smith (1997), and HK2000. We confirm the previous estimates of the mass-loss rate, but obtain a higher luminosity of log  (L/L) = 5.65, which can be partly attributed to the hotter stellar temperature derived from our line fit. In agreement with the classification, we find a hydrogen mass-fraction of XH = 0.3, which is 0.1 dex lower than previously derived by HK2000.

BAT99 31

was classified as WN4b binary candidate by Foellmi et al. (2003b). However, these authors could not derive a period from their radial velocity measurements. As we find no indications of a companion in its spectrum, we consider the contribution to the bolometric luminosity of this possible companion as negligible and analyze this object as a single WN star.

BAT99 32

is long known as a binary system with a short period of P = 1.91 d (Moffat 1989; Schnurr et al. 2008). It is listed with a WN6(h) spectral type in the BAT99 catalog. We note that emission lines like He iiλ 4686 and He iiλ 5412 exhibit an asymmetric line shape in the spectrum obtained by Torres-Dodgen & Massey (1988), whereas none of these peculiarities can be detected in the spectrum by Schnurr et al. (2008), which might be an effect of the lower spectral resolution. Since we cannot with certainty estimate the flux contribution of the companion, we consider the derived physical parameters of this object as uncertain.

Fairly similar stellar parameters were obtained by Koesterke et al. (1991), Crowther & Smith (1997), and HK2000, although the derived stellar temperatures exhibit some scatter. Our analysis, on the other hand results in a stellar temperature of T = 47 kK, i.e., 7 kK higher than the highest temperature obtained in previous studies. Similar to BAT99 30, the high temperature derived in our analysis probably originates from the line-blanketing effect included in our code. Since the earlier analyses by Koesterke et al. (1991) and Crowther & Smith (1997) are based on models that do not account for wind inhomogeneities, the mass-loss rate derived by these authors is certainly overestimated. However, the scaling of these values according to the clumping factor D = 10 assumed in this work results in nearly the same mass-loss rate. The same is true for the results obtained by HK2000, who assumed a lower clumping factor of D = 4. Compared to Koesterke et al. (1991), Crowther & Smith (1997), and HK2000, we have derived a higher luminosity mainly due to the different stellar temperature obtained in our analysis.

BAT99 33

belongs to the category of Ofpe/WN9 stars (also designated as “cool slash-stars”), that were incorporated in the WN-subclass system (Crowther et al. 1995a; Crowther & Smith 1997) as WN9–11 stars. According to Crowther & Smith (1997), this star is exceptional, since its spectral peculiarities prevent a closer classification. A detailed discussion of the spectral morphology can be found in Crowther & Smith (1997) and Pasquali et al. (1997). Humphreys & Davidson (1994) list this star as a LBV candidate. No clear evidence for a circumstellar nebular was found by Weis (2003). They attributed the nebulosity previously detected by Nota et al. (1996) to a background H ii region due to the low densities derived by these authors. In contrast Gvaramadze et al. (2010) found a bow-shock structure to the east of BAT99 33, using archival 24 μm data obtained with the Multiband Imaging Photometer (MIPS) aboard the Spitzer Space Telescope. Significant intrinsic line polarization was discovered by Vink (2007), suggesting the presence of an asymmetric stellar wind, which could explain the peculiarities visible in the spectrum of this star.

Pasquali et al. (1997) inferred a stellar temperature of T = 35 kK, which is 7 kK higher than the temperature obtained in our analysis. Our 35 kK model completely overestimates the strength of the He iiλ 4686,5412 lines in comparison to the He i lines, which unambiguously points to lower stellar temperatures. Moreover, the former authors speculated that the He i and He ii lines originate from different wind components because the helium line widths do not follow their expectations. Thus, they noted that their stellar parameters may not accurately represent the physical conditions of this star. With the exception of the asymmetric He iiλ 4686 line, the line width of all helium lines can be consistently reproduced in our analysis.

BAT99 35

was classified as WN3(h) by Foellmi et al. (2003b). Our analysis results in similar stellar parameters compared to the previous analysis by HK2000. The only exception is the inferred stellar temperature, which is reduced by one grid step in our present study, leading to a slightly better reproduction of the He i/He ii ratio. The best fit is obtained with a hydrogen mass-fraction of XH = 0.1, although a hydrogen-free atmosphere cannot be excluded with certainty.

BAT99 36

is one of two WNE/WCE transition type stars in the LMC that exhibit a clear carbon enhancement compared to the rest of the LMC WN sample. As noted by Foellmi et al. (2003b), the top of the Balmer lines are attenuated, which, according to these authors, could be due to a faint absorption-line companion, as already proposed by Crowther et al. (1995b). However, Foellmi et al. (2003b) note that their radial velocities measurements are consistent with a single star. We regard this object as a binary suspect until more appropriate data are available. Considering the small effect on the emission lines of the WN star, we expect that the potential companion does not contribute much to the bolometric luminosity.

We obtained quite the same stellar parameters as HK2000. Analyses prior to HK2000 suggested lower values for the stellar temperature and luminosity but nearly the same mass-loss rate (Koesterke et al. 1991; Crowther et al. 1995b). With a carbon abundance of XC = 0.003 derived in our analysis, we can confirm the previous results obtained by Crowther et al. (1995b).

BAT99 37

has the spectral type WN3o (Foellmi et al. 2003b). Unfortunately, we do not have an interpretable UV spectrum of this star. The optical spectrum from Torres-Dodgen & Massey (1988) is very noisy, so that reddening and luminosity cannot be determined precisely. Fitting this spectrum, the color excess is Ebv = 0.7 mag, while a color excess of Ebv = 0.5 mag is necessary to obtain a good fit of the photometry measured by Crowther & Hadfield (2006); the luminosity does not differ between the fits.

BAT99 40

is listed as WN5o+O binary candidate in the BAT99 catalog. However, Foellmi et al. (2003b) did not find a radial velocity period and reclassified this star to WN4(h)a. Furthermore, they attributed the absorption lines visible in their spectra to be intrinsic to the wind of the WN star. These authors, on the other hand, have detected X-ray emission in the archival ROSAT data, whereas Guerrero & Chu (2008b) list this object as undetected by ROSAT observations. Despite these contradicting results, we treat this star as a binary suspect until its binary status is clarified. The stellar parameters given in this work are the first derived for this object.

BAT99 41

is a WN4b star (BAT99) that has never been analyzed before. We note that this stars falls into the regime of parameter degeneracy (cf. Sect. 4.2).

BAT99 42

is the brightest source in our sample, with an extremely high stellar luminosity of log  (L/L) = 8.0. This is 1.06 dex higher than the stellar luminosity derived by Crowther et al. (2010) for BAT99 108 in the core of R136, and 1.2 dex higher than the value obtained by Sana et al. (2013b) for the binary BAT99 118 (R144).

BAT99 42 is a visual binary classified as WN5b(h) (Foellmi et al. 2003b) with a long known B-type supergiant (B3I) companion (Smith et al. 1996, and references therein). For the whole system, the detection of X-ray emission was reported by Guerrero & Chu (2008b). Seggewiss et al. (1991) found a spectroscopic period of P = 30.18 d, though this value is highly uncertain, according to the authors. Foellmi et al. (2003b), on the other hand, concluded from their radial velocity studies that the WN component is probably not a spectroscopic binary, although some scatter is present in their radial velocity data.

This object is associated with the LH 58 cluster, which is located about 1.1° to the northwest of 30 Doradus. New UV observations of this cluster were obtained by P. Massey with the WFC3 aboard the HST. These high resolution images dissolve BAT99 42 in three major components (see Fig. B.1) that lie within the slit width used by Foellmi et al. (2003b) and Torres-Dodgen & Massey (1988) (see Fig. B.1).

Figure B.1 even shows a small cluster around BAT99 42. This whole cluster is completely covered by the large aperture of the IUE satellite, which was used for the UV observations analyzed in this work. Thus, all available spectra represent at least the three major objects in this small cluster. A disentanglement of the spectra is beyond the scope of this paper and will be the subject of our forthcoming work. The photometry used to construct the observed SED is affected by the same problem since the core of this cluster cannot be resolved by most of the available instruments.

Apart from the lower signal-to-noise ratio (S/N), the flux-calibrated spectrum obtained by Torres-Dodgen & Massey (1988) is almost identical to the spectrum observed by Foellmi et al. (2003b). The weak round-shaped emission lines are obviously diluted by the contribution of the non-WN components to the overall flux. The round shape of the emission lines requires a convolution of the model spectrum with a rotation profile corresponding to a rotational velocity of vrotsini = 2300 km s-1. Thus, the WN component might belong to the so-called “round line” stars.

The narrow absorption lines from the non-WR components of this system are clearly visible in the high S/N spectrum from Foellmi et al. (2003b). These lines are relatively weak, which can be attributed to dilution effects as well. Therefore, we estimate the contribution of the WN component to the bolometric luminosity to be considerable although the non-WN components probably contribute most to the flux in the optical spectral range. Thus, this cluster probably hosts one of the most luminous WN stars in the LMC. However, without additional data it is hard to constrain the real luminosity of the WR component.

thumbnail Fig. B.1

An HST-WFC3 F225W image of the region within about 10″ distance of BAT99 42. The image was requested from the HST archive. The cycles refer to the apertures of the WISE photometer (6.1″ for the W1 and 12″ for the W4 band, respectively) and the maximum slit width used by Foellmi et al. (2003b).

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BAT99 43

was listed as WN4+OB binary candidate in the BAT99 catalog. Foellmi et al. (2003b) found a 2.8 d period for this double-line spectroscopic binary (SB2; Moffat 1989). We expect that the companion does not contribute much to the bolometric luminosity of this binary system (see also Foellmi et al. 2003b), since we do not see any unambiguous features of the companion in our spectra. This object has been previously analyzed by Koesterke et al. (1991) and HK2000. These authors have derived a stellar temperature of T = 79.4 kK, one grid step above our new results. A higher stellar temperature in our analysis, however, would result in a complete mismatch of the observed N iv / N v ratio. To the contrary, the line strength of the He iλ 5877 line points to an even lower stellar temperature. Compared to HK2000, the luminosity obtained here is almost a factor of two lower, while the mass-loss rate is approximately the same.

BAT99 44

was classified as WN8ha by Schnurr et al. (2008). Bonanos et al. (2009) discovered an infrared excess by comparing the observed SED with line-blanketed atmosphere models (Smith et al. 2002), which exhibit a stellar temperature of T = 45 kK. This temperature is considerably higher than obtained by Crowther et al. (1995b), using unblanketed model atmospheres. Our analysis agrees with the estimates by Bonanos et al. (2009). However, we cannot find an infrared excess for this object. Compared to Crowther et al. (1995b), the luminosity obtained in this work is a factor of two higher, mainly due to the higher stellar temperature, while the mass-loss rate is fairly the same.

BAT99 46

is a WN4o star (BAT99), which was not spectroscopically analyzed by means of model atmospheres before.

BAT99 47

is classified as WN3b (Foellmi et al. 2003b) and had never been analyzed before. Although Foellmi et al. (2003b) could not find periodic radial velocity variations, we treat this object as a binary suspect because of the X-ray emission reported by Guerrero & Chu (2008b). Unfortunately, we do not have flux-calibrated spectra for BAT99 47. However we have photometric data from the UV to the mid-infrared, so that the luminosity and the interstellar reddening can be well determined. We achieved a plausible fit of the SED with our single-star model. Therefore, we expect that the possible companion does not contribute much to the bolometric luminosity.

BAT99 48

is listed as a WN4b in the BAT99 catalog. This star was previously analyzed by Koesterke et al. (1991) and HK2000. The former authors have obtained a stellar temperature of T = 57 kK based on unblanketed model atmospheres, whereas the latter authors derived a stellar temperature of T = 79.4 kK with blanketed model atmospheres. In comparison to the last work, our new analysis results in a 10 kK higher stellar temperature, a factor of two higher mass-loss rate but nearly the same luminosity. We note that this star is located within the regime of parameter degeneracy (cf. Sect. 4.2).

BAT99 49

was identified as a SB2 binary with a period of 34 d (Niemela 1991). A slightly smaller period of 31.7 d was found by Foellmi et al. (2003b), who classified the primary as WN4:b and the companion as O8 V. The stellar parameters that we derived for this object are to be taken with care, since we analyze this object as a single star but the companion may substantially contribute to the overall flux.

BAT99 49 is one of only two WR stars in the LMC that is able to ionize a He ii region (Nazé et al. 2003b): the other one is BAT99 2. Nazé et al. (2003b) conclude from the nebular He ii-flux that the exciting star delivers ~1 × 1047 He ii ionizing photons per second. This agrees with our final model, which produces 1.6 × 1047 He ii ionizing photons per second. However, we note that our final model probably overestimates the number of ionizing photons, since the proper luminosity of the WN component is certainly lower than the value given in Table 2, which is derived neglecting its binary nature.

BAT99 50

was classified as WN5h by Crowther & Hadfield (2006). For this object, we have derived the stellar parameters by means of stellar atmosphere models for the first time. Unfortunately, we do not have UV spectra so the stellar parameters are derived from this normalized optical spectrum and photometry alone. Thus, the obtained luminosity is subject to higher uncertainties compared to those luminosities simultaneously derived from flux-calibrated spectra and photometry.

BAT99 51

is listed with a WN3b classification in the BAT99 catalog. The spectrum of BAT99 51 is dominated by broad and round emission lines. Therefore, this object belongs to the so-called round-lined stars (cf. comment on BAT99 7). The line shapes can only be reproduced assuming a high rotational velocity of vrot = 1000 km s-1 as discussed in Sect 4.3.

Unfortunately, we do not have an interpretable UV spectrum for this stars. Its only IUE spectrum (SWP 04872) is not usable. It is noisy and lacks WR features, and does not show enough flux in relation to the photometric data and flux-calibrated spectrum obtained by Torres-Dodgen & Massey (1988).

BAT99 54

was classified as WN8ha by Schnurr et al. (2008). Unfortunately, the optical spectra at hand do not cover the Hα line. Consequently, the hydrogen abundance is determined from the higher members of the Balmer series alone. We note that the line strength of the Hβ line may point to a slightly higher hydrogen mass-fraction than the XH = 0.2 given in Table 2. With our line-blanketed models we derived a 7 kK higher stellar temperature than previously obtained by Crowther & Smith (1997) with unblanketed model atmospheres. This higher value results in a roughly 50% higher luminosity, whereas the mass-loss rate is a factor of three higher in our analysis.

BAT99 55

is one of only three WN11 stars in the whole sample. According to Humphreys & Davidson (1994), this WN star is a LBV candidate. Schnurr et al. (2008) concluded from the radial velocity of this object that it is most likely a runaway star. BAT99 55 is one of three LMC WN stars detected at 24 μm with the IRAC instrument aboard the Spitzer space telescope, suggesting the presence of circumstellar dust (Bonanos et al. 2009). Crowther & Smith (1997) and Pasquali et al. (1997) derived rather similar stellar parameters for this star. The stellar temperature and the luminosity derived in this work are in good agreement with the properties presented by Crowther & Smith (1997). On the other hand, our study results in a 70% higher mass-loss rate, which can be attributed to the higher terminal wind velocity derived in our analysis.

BAT99 56

is a WN4b star (BAT99). This object had never been spectroscopically analyzed by means of model atmospheres before.

BAT99 57

is another WN4b star (BAT99) with typical stellar parameters, which was not analyzed by means of model atmospheres before.

BAT99 58

is a WN7h star (Schnurr et al. 2008) studied by Koesterke et al. (1991), Crowther & Smith (1997), HK2000 and Bonanos et al. (2009). Koesterke et al. (1991) and Crowther & Smith (1997) derived nearly equal stellar parameters, including a stellar temperature of about T = 35 kK. HK2000 obtained a stellar temperature of T = 39.8 kK. Our state of the art atmosphere models, however, need a temperature of T = 47 kK to reproduce the observed He i/He ii line ratios. These differences are attributable to the line-blanketing effect included in our models. The higher stellar temperature contributes to an increase in the derived luminosity of about 0.5 dex compared to Koesterke et al. (1991), Crowther & Smith (1997), and HK2000. The derived mass-loss rate, on the other hand, is fairly the same.

Bonanos et al. (2009) find an infrared excess for this object by comparing a T = 45 kK CMFGEN model from Smith et al. (2002) with the observed SED. However, our fit does not show a clear infrared excess, although a slight mismatch of the infrared photometry can be seen in Fig. C.23. We note that this discrepancy may be attributed to the IUE spectra, which are of poor quality (bad S/N, arbitrary continuum shape of the IUE long-wavelength spectrum).

BAT99 59

is listed as WN4o?+B binary candidate in the BAT99 catalog. Foellmi et al. (2003b) found a period of 4.7 d, reclassified the primary to WN4b and specified the companion to be an O8: star. They still assign a question mark to the binary status because the determined radial velocity amplitude is close to their detection limit. Since distinct absorption lines of the companion are visible in the spectra, the companion should contribute substantially to the total flux. The derived stellar properties (Table 2) are thus to be considered with caution. Koesterke et al. (1991) used pure helium models to derive lower limits for the stellar temperature and luminosity than our more sophisticated models.

BAT99 60

is listed as WN3+OB binary candidate in the BAT99 catalog, but Foellmi et al. (2003b) did not find a radial velocity period and reclassified the object to WN4(h)a. Furthermore, these authors identified the absorption lines visible in their spectra to be intrinsic to the wind of the WN star. Therefore, we consider this star to be single for the time being. Unfortunately, we do not have UV spectra of this star, which had never been analyzed with stellar atmosphere models before. We confirm the presence of hydrogen (XH = 0.2) and thus the above classification.

BAT99 62

was classified as WN3(h) by Foellmi et al. (2003b) due to indications of hydrogen in its spectrum. Our analysis is inconclusive at this point. A reasonable fit can be achieved with a hydrogen-free model as well as with a model of a moderate hydrogen mass-fraction of XH = 0.1. All optical spectra at hand exhibit distinct absorption lines in place of the O iii-nebular emission lines at 4959 Å and 5007 Å, likely caused by an over correction of the diffuse emission. If this is true, the Balmer series will probably be narrowed by the inadequate nebular subtraction, lending credence to the relatively high hydrogen abundance derived in the line fit. In this paper, we present for the first time stellar parameters for this star, derived by spectral analysis. Regrettably, no UV spectra are available for this star.

BAT99 63

is listed as a binary candidate in the BAT99 catalog on the basis of absorption lines possibly belonging to a companion star. However, no binary period was found by the radial velocity analysis of Foellmi et al. (2003b). Moreover, Cowley et al. (1984) and Foellmi et al. (2003b) argue that the absorption components are intrinsic to the WR wind. Thus, the object was classified as WN4ha: by Foellmi et al. (2003b). We treat this star as single until its binary status is confirmed. Cowley et al. (1984) note that the high radial velocity suggests that the star is a runaway. This conclusion is confirmed by the radial velocity study of Schnurr et al. (2008).

A lower limit for the stellar temperature was derived by Koesterke et al. (1991) on the basis of the helium spectrum. More elaborate models are used by HK2000, deriving a stellar temperature of T = 70.8 kK. In comparison to this work, our best fitting model with T = 63 kK is one grid step cooler. This lower temperature is justified by a slightly better fit of the nitrogen lines. Apart from that, we obtained similar values for the mass-loss rate, while the luminosity and hydrogen abundance are slightly lower.

The filamentary ring nebula associated with BAT99 63 was studied by Nazé et al. (2003b). As no He ii nebular emission is detected by these authors, they obtained an upper limit for the number of He ii ionizing photons delivered by the exciting star, which amounts to <2.5 × 1045 He ii ionizing photons per second. This agrees with our final model, which does not predicts a significant number of He ii ionizing photons (see Table A.3).

BAT99 64

was listed as WN4+OB? binary candidate in the BAT99 catalog. A period of 37.6 d was found by Foellmi et al. (2003b), who classified the companion as O9 and specified the primary to be a WN4o star. However, we expect that the companion does not contribute much to the bolometric luminosity.

BAT99 65

was classified as WN4o by Foellmi et al. (2003b). For this star we only have a co-added optical spectrum, which is not flux-calibrated so that the observed SED is covered by photometry alone. However, the coverage is complete from UV to mid-infrared.

BAT99 66

is a WN3(h) star (Foellmi et al. 2003b) for which stellar parameters have been presented for the first time in this paper. Unfortunately, no UV spectra are available for this star.

BAT99 67

is listed as a binary candidate with a WN5o?+OB classification in the BAT99 catalog, though Foellmi et al. (2003b) concluded from their radial velocity study that this star is probably not a short-period binary. Foellmi et al. (2003b) note that the absorption features visible in their spectra are strongly blue-shifted. They emphasize that if this shift arises from a companion, it would result in distinct radial velocity variations that are not observed. Hence, these authors argue that the absorption lines are intrinsically formed in the wind of the WN star, and accordingly reclassify the object to WN5ha. Nevertheless, we still treat this star as a binary suspect because of the X-ray emission detected by Guerrero & Chu (2008a).

We derived a stellar temperature of T = 47 kK, only slightly higher than the value obtained by HK2000 (T = 44.7 kK). The luminosity and mass-loss rate derived in our analysis are slightly lower compared to the result obtained by HK2000, while the hydrogen abundance is 0.1 dex higher.

BAT99 68

is a transition type O3.5 If*/WN7 star (Crowther & Walborn 2011). The physical parameters were previously estimated with FASTWIND by Massey et al. (2005), but the authors note that these values are only poorly constrained. Our new analysis results in a stellar temperature of Teff = 44 kK, marginally higher than the temperature estimate obtained by Massey et al. (2005, Teff = 40 − 42 kK. The mass-loss rate derived in our new analysis is a factor of three lower, even if we account for the impact of different terminal velocities and density contrasts. We note that a mass-loss rate as high as obtained by Massey et al. (2005) results in considerably too strong emission lines in our synthetic spectra.

The IUE spectra associated with this object are noisy and do not show WR features. Therefore, we precluded these spectra from our analysis. We also excluded the optical spectrum measured by Torres-Dodgen & Massey (1988), since the continuum slope of this flux-calibrated spectrum disagrees with the optical photometry (BAT99) and the HST spectra. Moreover, the available infrared photometry (Cutri et al. 2012; Kato et al. 2007) is inconsistent and cannot be fitted in accordance with the UV and optical data. The infrared observations probably suffer from contributions of nearby objects, which are visible on WFPC2 images within 1″ around BAT99 68. Fitting the UV and optical data, we derived a luminosity of log  (L/L) = 6.0 which is a factor of two lower than previously obtained by Massey et al. (2005).

BAT99 71

was identified as a short-period binary by Foellmi et al. (2003b). They classified the primary as WN4 and specified the companion to be a O8: star. Its IUE spectrum is not usable: it is noisy and does not have distinct WR features. Therefore, we reject these spectra from our analysis.

BAT99 72

is probably a medium-period binary according to Foellmi et al. (2003b), assigning an O3: type to the companion and reclassifying the primary to WN4h. Foellmi et al. (2003b) mention that an absorption-line component is clearly visible in the spectrum. However, we fit the star as a single WN star and achieve a good fit for almost the whole spectrum. For that reason, and due to the well-reproduced SED, we expect that the companion does not contribute much to the bolometric luminosity. We note that the luminosity obtained in our analysis is derived from visual narrowband, 2MASS and IRAC photometry alone since no flux-calibrated spectra were available.

BAT99 73

was classified as WN5ha by Crowther & Hadfield (2006). Unfortunately, the spectrum from Foellmi et al. (2003b) is the only one available to us. Since this spectrum is not flux-calibrated, the luminosity is derived from photometry alone, but the available photometry covers the whole range from the UV to mid-infrared so that the luminosity is nevertheless well constrained. This WN star was not analyzed by means of stellar atmosphere models before.

BAT99 74

is a WN3(h)a star (Foellmi et al. 2003b), which is analyzed in this work for the first time. The spectrum of this star is characterized by small absorption lines and moderate emission lines. Foellmi et al. (2003b) identified the absorption lines to be intrinsic to the wind of the WN star. Both absorption and emission lines can be accordantly reproduced with our single-star model. From the Hα and Hβ lines, we estimated a hydrogen mass-fraction of XH = 0.2, which underpins the WN(h) classification. No flux-calibrated spectra are available for BAT99 74. However, the luminosity is well constrained due to the photometric coverage of the SED from UV to mid-infrared.

BAT99 75

was classified as WN4o by Foellmi et al. (2003b). The prominent emission lines in the spectra of this star are best reproduced with a hydrogen-free model. For the first time, we present here parameters for this star.

BAT99 76

is a WN9ha star (Schnurr et al. 2008), which has been analyzed by Koesterke et al. (1991), Crowther et al. (1995b), Pasquali et al. (1997), and HK2000. Two UV spectra and three optical spectra are at hand for the analysis of this star (see Table A.1 and A.2). All can be fitted with the same model, which give us confidence in the derived physical parameters.

The stellar temperature derived in our analysis, T = 35 kK, fits in the range of temperatures T = 30 − 38.7 kK obtained by the authors mention above. The mass-loss rates presented in these papers are nearly the same with the exception of the results obtained by HK2000, who inferred a mass-loss rate a factor of two higher. Our analysis confirms the lower mass-loss rates obtained by Koesterke et al. (1991), Crowther et al. (1995b), and Pasquali et al. (1997). The luminosity derived in the previous papers range from log  (L/L) = 5.4 to log  (L/L) = 5.8, which in principle agrees with the value (log  (L/L) = 5.66) derived in this work. The hydrogen abundance provided in Table 2 is lower than previously derived by Crowther et al. (1995b), Pasquali et al. (1997), and HK2000.

BAT99 77

was revealed as SB1 binary with a period of 3 d by Moffat (1989). This finding was later confirmed by the radial velocity study of Schnurr et al. (2008). These authors classified the object as WN7ha. Since no spectral features can be unambiguously attributed to the companion, the contribution of the companion to the overall flux is hard to estimate. Thus, the physical parameters listed in Table 2 need to be taken with caution. A tentative detection of X-ray emission has been reported by Guerrero & Chu (2008a).

BAT99 78

is located in a tight cluster in the western part of LH90 with several additional sources within a radius of less than 1″ (Walborn et al. 1999). This star is listed as WN4 in the BAT99 catalog, whereas Foellmi et al. (2003b) assign a WN6 spectral type to it. According to these authors, the spectrum is a superposition of the WN star and nearby objects. This is attributable to the crowded environment and the relatively wide slit width used by these authors. The co-added spectrum observed by Foellmi et al. (2003b) resembles an Of supergiant rather than a WN star since it shows distinct Of features and a Hβ line clearly in absorption (Crowther & Walborn 2011). A reasonable fit of this spectrum can be achieved with a model of T = 45 kK.

Beside the spectrum obtained by Foellmi et al. (2003b) we retrieved a relatively short optical spectrum from the HST archive which exhibits a clear WN characteristic. This spectrum can be well fitted with a model corresponding to a stellar temperature of T = 71 kK. Since the HST spectrum is flux-calibrated, it can also be used for the SED fit. This results in an implausibly small luminosity of log  (L/L) = 4.64. The flux of the HST spectrum disagrees with the available photometry, which in turn is not uniform. For example, the data of the J-band magnitude varies from 11.6 − 14.68 mag. A fit of the 2MASS, Spitzer IRAC, and WISE photometry results in a luminosity of log  (L/L) = 6.8. The observational discrepancies might be traced back to the small angular distance of BAT99 78 to the other cluster members (see, e.g., Walborn et al. 1999). The smallest WISE aperture covers not only the four closest objects to BAT99 78, but rather a large part of the whole cluster around it.

Walborn et al. (1999) were able to chiefly resolve the host cluster of BAT99 78 with the Wide Field/Planetary Camera 1 (WFPC1) aboard the HST. They obtained visual broadband photometry, which we adjusted to account for the contribution of the emission lines, using the correction factor derived by Breysacher (1986). This visual magnitude Mv = 14.84 results in a luminosity of log  (L/L) = 5.7, assuming that the continuum slope of the HST spectrum and the thereof derived color excess of Ebv = 0.2 is correct.

Due to the X-ray emission detected by Guerrero & Chu (2008a), we treat this object as a binary candidate, although Foellmi et al. (2003b) find no significant periodicity in their radial velocity study, which is based on a small number of observations of this object.

BAT99 79

is listed as WN7h+OB binary candidate in the BAT99 catalog. However, Schnurr et al. (2008) do not find periodic radial velocity variations. Despite this nondetection, we still regard this object as a binary candidate since a considerable amount of X-ray emission has been detected by Guerrero & Chu (2008a). We note that Crowther & Smith (1997) estimated a significant contribution of the companion to the overall flux by means of their own spectra. In contrast, we expect that the companion does not contribute much to the bolometric luminosity, since only very small absorption features can be seen superimposed on the emission lines of the WN star in the spectrum observed by Schnurr et al. (2008). Therefore, the stellar parameters listed in Table 2 should represent a reliable approximation.

BAT99 80

is listed as O4 If/WN6 transition type star in BAT99 catalog. Schnurr et al. (2008) reclassified the star to WN5h:a, arguing that the spectrum shows a diluted WN star rather than a hot slash star. However, no significant radial velocity variations were detected by Schnurr et al. (2008). Nevertheless, we treat this object as a binary suspect because of the X-ray emission reported in Guerrero & Chu (2008a).

The only spectrum available to us is characterized by absorption and moderate emission lines. Although it might show a binary, both absorption and emission lines can be accordingly reproduced with our single-star model.

BAT99 81

was classified as WN5h by Foellmi et al. (2003b). The IUE long-wavelength spectrum of this star apparently has a considerably lower flux than the IUE short-wavelength spectrum. The former does not fit to the rest of the observed SED. Therefore, it is not further considered in our analysis of BAT99 81, which provides stellar and wind parameters of this object for the first time. Foellmi et al. (2003b) highlights the differences between the results of Cowley et al. (1984) and their own radial velocity study, which exhibits no clear radial-velocity deviation from the mean vrad of their WN sample, whereas Cowley et al. (1984) argued in favor of a runaway nature.

BAT99 82

is a WN3b star (BAT99), for which the detection of X-rays has been reported by Guerrero & Chu (2008a). We treat this star as a binary on the basis of the X-ray emission. However, no periodic radial velocity variations were found by Foellmi et al. (2003b). The IUE spectrum assigned to BAT99 82 was taken 1.2′ away from its position and does show some helium emission, but no nitrogen. Furthermore, its flux is not compatible to the SED composed of the optical spectrum by Torres-Dodgen & Massey (1988) and the photometry from the 2MASS catalog, so that the IUE spectrum was ignored.

BAT99 86

has recently been classified as WN3(h) by Doran et al. (2013). Unfortunately, no UV spectrum is available for this object. The hydrogen abundance is hard to determine because of absorption features superimposed on the Balmer lines. The best fit is obtained with a hydrogen-free model. However, we note that the emission lines without the absorption components might agree with an hydrogen mass-fraction of XH = 0.1. On the contrary, Doran et al. (2013) derived a hydrogen abundance of XH = 0.2 on the basis of their stellar atmosphere models. However, a hydrogen abundance this high would result in a considerable overprediction of the Balmer lines by our synthetic spectra. Apart from that, Doran et al. (2013) derived a stellar temperature that is 8 kK higher and a luminosity that is 0.1 dex higher than our values. However, the strongest deviation to the results presented by Doran et al. (2013) is obtained in the derived mass-loss rate, which is more than nine times lower in our analysis. Our models with a mass-loss rate as high as derived by Doran et al. (2013) considerably overpredict the equivalent width of all emission lines.

BAT99 88

is the second WNE/WCE transition type star in the LMC. These stars exhibit a clear carbon enrichment compared to the rest of the LMC WN stars. The best fit is obtained with a carbon mass-fraction of XC = 0.005 in accordance with the estimate by Doran et al. (2013).

This is one of the hottest stars in our sample, with a stellar temperature of T = 112 kK. This value is about 30 kK higher than the stellar temperature (T = 80 kK) obtained by Doran et al. (2013). In contrast, our model with T = 80 kK underpredicts the equivalent width of the He iiλ 4686 line with respect to the other He ii lines, while the model with T = 112 kK is able to simultaneously reproduce the equivalent width of all He ii lines. Furthermore, the N iv and N v lines are slightly better reproduced at this higher temperature. However, we note that this star is located within the regime of parameter degeneracy (cf. Sect. 4.2).

Contrary to the stellar temperature, the luminosity and the mass-loss rate derived in this work are comparable to the values derived by Doran et al. (2013). Unfortunately, we neither have UV spectra nor flux-calibrated spectra. Thus, the luminosity is derived from UV, optical, near- and mid-infrared photometry alone.

The emission lines of this object exhibit round profiles. Thus, this star falls in the category of the so-called round line stars (cf. comment on BAT99 7). A reasonable fit of the observed line profile is achieved by convolving our synthetic spectrum with a rotation profile corresponding to a rotational velocity of vrotsini = 1200 km s-1.

BAT99 89

is a WN7h star (BAT99), which was previously studied by Crowther & Smith (1997) and Doran et al. (2013). The best fit is achieved at a stellar temperature of T = 50 kK, which agrees well with the recent results of Doran et al. (2013). In contrast, Crowther & Smith (1997) obtained a stellar temperature that is 10 kK lower. However this value is excluded by our analysis, since the synthetic spectra clearly overpredicts the He i/He ii ratio at this temperature. This difference in the stellar temperature can be attributed to the line-blanketing effect, which is included in modern atmosphere models, but was not regarded at the time of the study by Crowther & Smith (1997). The luminosity derived in this paper (log  (L/L) = 5.56) is only marginally higher than the value obtained by Doran et al. (2013), while Crowther & Smith (1997) derived a luminosity which is a factor of two lower. In terms of the mass-loss rate, we obtain a value of almost 50% lower compared to the recent results by Doran et al. (2013), whereas the mass-loss rate derived by Crowther & Smith (1997) is slightly higher.

BAT99 91

was resolved into multiple components by Testor et al. (1988) and later by the HST observations of Walborn et al. (1995). The WN star has been classified as WN6(h) by Evans et al. (2011), although their ground-based observations were not able to entirely resolve the components of this object. The same seems to hold for the spectrum obtained by Schnurr et al. (2008), which does not resemble the HST spectrum for BAT99 91 obtained by Walborn et al. (1995). For that reason, our spectroscopic analysis relies on the HST spectrum alone, which unfortunately covers only 1500 Å out of 3300–4800 Å. Since this range comprises neither the Hα nor Hβ, the determination of the hydrogen abundance is based on the higher members of the Balmer series alone. With a hydrogen mass-fraction of XH = 0.2, we confirm the corresponding classification. In the SED fit, we exclusively used the HST photometry from Walborn et al. (1995) and the HST flux-calibrated spectra from Walborn et al. (1999), since these observations are distinguished by their particularly high spatial resolution, which seems to be necessary for reliable results in this tight cluster. This star had never been analyzed before.

BAT99 92

was classified as WN3:b(+O)+B1 Ia by Schnurr et al. (2008). Both the BAT99 catalog and Schnurr et al. (2008) give a binary period of 4.3 d for this star. We note that the detection of X-ray emission has been reported by Guerrero & Chu (2008a). Although the optical spectra seem to be considerably affected by the companions, they still allow us to assess parameters like the stellar temperature. Furthermore, this object belongs to the round-line stars, since the emission lines exhibit a round shape which can only be reproduced with a rotational velocity of vrot = 1500 km s-1. Another interesting fact is the significant strength of the C ivλ 5808 emission line, suggesting that either the system comprises an additional WC star or that the WN star belongs to the rare WNE/WCE transition type. The SED can be well reproduced with a single-star model.

BAT99 93

is one of the stars listed as WN stars in the BAT99 catalog which has been downgraded to O3 If* by Evans et al. (2011) and Crowther & Walborn (2011). Tentative X-ray emission has been detected by Guerrero & Chu (2008a). Thus, we treat this object as a binary suspect, although no radial velocity variations have been detected by Schnurr et al. (2008). The only optical spectrum at hand lacks a subtraction of the diffuse background and shows only a truncated Hβ line. For these reasons, we are not able to give a precise hydrogen abundances for this star.

BAT99 94

is characterized by broad emission lines with a round line shape. Therefore, this WN4b star (BAT99) is classified as a round line star (cf. comment on BAT99 7). The round shape of the emission lines requires a convolution of the model spectrum with a rotation profile corresponding to a rotational velocity of vrot·sini = 1600 km s-1.

Unfortunately, we do not have UV spectra for this star. In this work, we derive a stellar temperature of T = 141 kK, which is significantly higher compared to the previous results by HK2000 (T = 100 kK). In our analysis, the model with the higher temperature results in a sightly better fit of the He iiλ 4686, C ivλ 5808 and N ivλ 4060 lines. However, we note that star is located in the regime of parameter degeneracy (cf. Sect. 4.2). In comparison with HK2000, we have obtained a factor of two lower mass-loss rate, while the luminosity is a factor of three higher. The higher luminosity originates from the higher temperature and thus higher bolometric correction.

BAT99 95

was identified as binary by Schnurr et al. (2008). These authors find radial velocity variations with a period of 2.1 d. Evans et al. (2011) classified the object as WN7h+OB. We expect that the companion does not contribute much to the bolometric luminosity, since we do not see spectral features of the companion in any of the available spectra. However, all optical spectra available to us suffer from either an oversubtraction of the diffuse background, or even a missing background subtraction. For this reason we cannot give a reliable value for the hydrogen content in the atmosphere of this star.

In comparison to the previous analysis by Crowther & Smith (1997), our best fit is achieved at a 14 kK higher stellar temperature of T = 50 kK. The temperature derived by Crowther & Smith (1997), however, would result in an overprediction of the He iλ 5877 to He iiλ 5412 line ratio. The different temperatures likely arise from the line blanketing, which is incorporated in our stellar atmosphere models, but was not accounted for in the models used by Crowther & Smith (1997). The higher temperature derived in this work results in a luminosity that is almost a factor of three higher compared to the previous results by Crowther & Smith (1997). The mass-loss rate derived in this work, on the other hand, is identical to the value given by Crowther & Smith (1997).

BAT99 96

is of subtype WN8 (Schnurr et al. 2008) and located in the southern part of 30 Doradus. We do not have flux-calibrated spectra or intrinsic narrowband photometry for this star, which renders it difficult to obtain precise values for the stellar luminosity and the interstellar reddening. Moreover, the available UBVR photometry is inconsistent since the values derived by various authors differ by up to 2 mag. The narrowband photometry listed in the BAT99 catalog is derived from visual broadband photometry (Parker 1993, V = 13.65 mag) by means of the correction factor found by Breysacher (1986). The visual magnitude obtained by Parker (1993) is slightly higher than the value (V = 13.76 mag) observed by Selman et al. (1999). Massey (2002) and Zaritsky et al. (2004) obtained higher magnitudes of V = 12.84 mag and V = 12.9 mag, respectively. In contrast, Duflot (2010) and Girard et al. (2011) derived lower visual magnitudes of V = 14.5 mag and 14.47 mag, respectively.

To construct the SED, we relied on the optical photometry obtained by Parker (1993), 2MASS and IRAC photometry (Bonanos et al. 2009). Similar to BAT99 98, the SED fit results in a relatively high value for the color excess (Ebv = 0.7 mag) and the luminosity (log  (L/L) = 6.4). In contrast to our results, Doran et al. (2013) derived a color excess of EBV = 0.65 mag, which corresponds to Ebv = 0.54 mag. We cannot achieve a reasonable SED fit with a color excess as low as derived by these authors. Moreover, we obtain the same color excess and luminosity in our SED fit regardless of whether the optical broadband photometry from Parker (1993) or the optical narrowband magnitudes (v = 13.82 mag, bv = 0.49 mag) given by Doran et al. (2013) are used.

The stellar luminosity derived in our analysis is much higher compared to the results derived by Crowther & Smith (1997) and Doran et al. (2013). These authors determined a stellar luminosity of log  (L/L) = 5.86 and log  (L/L) = 6.04, respectively. In comparison to Doran et al. (2013), the deviation in the luminosity primarily originates from the different reddening parameters. The additional IRAC photometry incorporated in our analysis enhances the constraints on the shape of the SED (see Fig C.38), which gives us confidence in the derived luminosity and color excess. Apart from these two parameters, we achieve a good agreement for the stellar temperature and the mass-loss rate obtained in this work and the values presented by Doran et al. (2013). Due to the missing subtraction of the diffuse background in the spectra obtained by Schnurr et al. (2008), we note the doubtful hydrogen abundance determined in our analysis.

We consider this star to be single, since neither periodic radial-velocity variations nor X-ray emission have been detected by Schnurr et al. (2008) and Guerrero & Chu (2008a,b), respectively. However, Parker (1993) argued in favor of a multiple object on the basis of their ground-based photometry. However, we cannot detect any visual companion either on the images taken with the Visual and Infrared Telescope for Astronomy (VISTA) for the VISTA survey of the Magellanic Clouds system (VMC; Emerson et al. 2006; Dalton et al. 2006) or on the high resolution images with the WFC3 aboard the HST (O’Connell 2008). From a comparison of the empirical HRD position (see Figs. 7 and 10) with the stellar evolution tracks calculated by Meynet & Maeder (2005), we estimate an initial mass on the order of Minit = 100 M. Thus, we consider this object to belong to the category of very massive stars.

BAT99 97

is another transition type O3.5 If*/WN7 star (Crowther & Walborn 2011; Evans et al. 2011). Unfortunately, the only spectrum available to us is affected by nebular emission due to a missing background subtraction. Thus, we are not able to give a precise hydrogen abundances for this star. We derive a stellar temperature that is slightly higher compared to the results published by Doran et al. (2013), while the mass-loss rate and the luminosity are higher by about 0.15 dex.

BAT99 98

is a WN6 star (Schnurr et al. 2008) located near R136. The derived luminosity, and thus the stellar mass as well, are comparable to those of the very massive stars in the core of R136 analyzed by Crowther et al. (2010). This star is distinguished by the relatively high extinction of Ebv = 0.8 mag derived from the SED fit. Unfortunately, no flux-calibrated spectra and intrinsic narrowband photometry (Smith system) are available. Broadband photometry (e.g., Johnson system), on the other hand, is contaminated by the prominent emission features.

The optical narrowband photometry from the BAT99 catalog is a corrected Johnson V magnitude from Parker (1993), using the subtype-dependent correction factor derived by Breysacher (1986). Following this procedure, we obtain a narrowband magnitude of v = 13.70 mag. Further broadband photometry is available from Selman et al. (1999) and Massey (2002), whereas the coordinates quoted by Massey (2002) show the largest deviation from the position stated in Simbad. The V band magnitudes obtained by these authors, corrected for the contribution of the emission lines, result in v = 13.64 mag, v = 13.61 mag and v = 13.67 mag, respectively. The v magnitude inferred by Breysacher (1986) is a corrected Strömgen y magnitude that amounts to v = 13.65 mag. The optical photometry, together with the near- and mid-infrared photometry, gives rise to the high color excess and a stellar luminosity of log  (L/L) = 6.7. However, the luminosity has a relatively large uncertainty, since it is derived from photometry alone.

So far, neither periodical radial velocity variations (Schnurr et al. 2008) nor X-ray emission (Guerrero & Chu 2008a,b) were detected. Thus, we treat this object as a single star, although the moderate fit quality may indicate a line dilution due to a yet undetected companion, as already suggested by Crowther & Smith (1997). The applied model underpredicts the He iiλ 5201, He iλ 4471 lines and overpredicts the He iiλ 4686 line. An adjustment of the temperature in one direction or the other spoils the fit of either the N ivλ 4060 line or the N iiiλ 4640 line, used as main diagnostic lines for this object.

Unfortunately, the only spectrum available to us is compromised by a missing subtraction of the diffuse background. This is probably the reason that the spectrum is affected by nebular contamination, a fact that entails an uncertain determination of the hydrogen content. This is aggravated by the fact that Hβ and Hγ are truncated (see Schnurr et al. 2008).

The luminosity derived in this work corresponds to a current mass of M = 226 M, according to the mass-luminosity relation from Gräfener et al. (2011). However, the error margin of this quantity is large, since it is calculated from the luminosity and the hydrogen abundance, which are in turn affected by considerable uncertainties. By comparing the empirical HRD position to stellar evolution tracks calculated by Yusof et al. (2013), we estimated an initial mass of at least Minit = 250 M. Thus, this star is one of the most massive stars hitherto known in the LMC.

BAT99 99

is a transition type O2.5 If*/WN6 star. Two spectra are used for the analysis of this star, an HST spectrum and a spectrum obtained by Schnurr et al. (2008). Both spectra are characterized by small absorption lines and relatively weak emission lines. We note that the spectrum taken by Schnurr et al. (2008) shows a substantially weaker He iiλ 4686 line, although the N ivλ 4060 line is of comparable strength in both observations. However, we do not consider the ground-based spectrum observed by Schnurr et al. (2008) in our analysis because it seems to be contaminated with nebular emission and the Hβ line is arbitrary truncated.

The HST spectra exhibit a flux of roughly a factor of two lower in comparison to the photometric data listed in the BAT99 catalog and the infrared photometry by Kato et al. (2007). Since BAT99 99 is located in the vicinity of the 30 Doradus core, this mismatch can be attributable to this crowded environment. Therefore, we have derived the luminosity from the HST spectra alone, which are distinguished by the high spatial resolution of the HST. The infrared excess presented in the SED fit in Fig. C.39 presumably originates from nearby sources visible on high resolution HST images.

The detection of X-ray emission has been reported by Guerrero & Chu (2008a), which is indirect evidence for the binary nature of this object (see Sect. 2.2). Direct evidence has been supplied by Schnurr et al. (2008). These authors have found radial velocity variations, corresponding to a period of 93 d. We did not detect spectral lines of a companion star in either of the spectra used in our analysis. Therefore, the contribution of an OB companion to the overall flux cannot be properly evaluated. Thus, the parameters for the WN component listed in Table 2 might be affected by this unknown flux contribution.

BAT99 100

is a WN7 star located in the crowded environment close to the core of 30 Doradus. In the optical spectral range, we used two spectra, an archival HST spectrum and a spectrum obtained by Schnurr et al. (2008). The latter shows strong nebular emission lines, but only small stellar emission lines, whereas the former exhibit much stronger emission lines. Due to the limited spatial resolution of the ground-based observations, our analysis is mainly based on the HST spectrum.

In a former study, Crowther & Smith (1997) analyzed this star with unblanketed stellar atmospheres. In contrast to their work, we achieve the best fit with a model corresponding to a stellar temperature of T = 47 kK, which is 15 kK higher than the temperature derived by these authors. At stellar temperatures below 47 kK, our models overpredict the He i/He ii line ratio. These differences can be attributed to the line-blanketing effect. The luminosity derived in this work is a factor of two higher compared to the results obtained by Crowther & Smith (1997), which is attributable to the different bolometric correction due to the higher stellar temperature. The mass-loss rate is nearly the same compared to the previous estimate by Crowther & Smith (1997). Since the HST spectrum does not cover the Hα and Hβ lines, the hydrogen abundance is derived from the higher Balmer members alone.

BAT99 100 was found by Guerrero & Chu (2008a) to show X-ray emission. Thus, we consider this object to be a binary suspect, although no radial velocity variations were discovered by Schnurr et al. (2008). The noteworthy infrared excess of this object might be attributed to a hidden companion.

BAT99 102

was classified as WN6 by Schnurr et al. (2008). According to these authors, the spectra are contaminated by the flux of the WC star BAT99 101, since it was not possible to resolve these close objects even under the best seeing conditions (Schnurr et al. 2008). Unfortunately, this is the only spectrum available to us. Since this spectrum lacks a sufficient background subtraction, we are not able to derive a reliable hydrogen abundance. Moreover, the flux contribution of BAT99 101 to the total flux is unknown but not negligible, since the broad line wings of the He iiλ 4686 line and He iiλ 5412 line (Fig. C.40) probably originate from the WC star. Therefore, the physical parameters listed in Table 2 need to be taken with caution.

According to Guerrero & Chu (2008a), BAT99 101, together with BAT99 102 is one of the brightest X-ray source in 30 Doradus. Unfortunately, the Chandra ACIS instrument is not able to resolve BAT99 101 and 102, so that the X-ray emission cannot be attributed to one of these stars alone. The ROSAT HRI observation analyzed by Guerrero & Chu (2008b) results in the same conclusion, even though BAT99 101–103 could not be resolved into individual objects by ROSAT. Moffat et al. (1987) found radial velocity variations with a period of 2.76 d for BAT99 102, whereas Schnurr et al. (2008) found the same period for the nearby BAT99 103 instead of for BAT99 102. Until the binary status is confirmed, we consider this star as a binary suspect.

BAT99 103

is a WN5(h)+O binary (Evans et al. 2011) located in the direct neighborhood of BAT99 101 and 102. This star was identified as a binary with a period of 2.76 d by Schnurr et al. (2008). Tentative X-ray emission was reported by Guerrero & Chu (2008a).

Unfortunately, no background subtraction was applied to the spectrum shown in Fig. C.41. Since this is the only spectrum available to us, we are not able to derive a meaningful hydrogen abundance for this object.

BAT99 104

is a O2 If*/WN5 transition type star located close to the center of 30 Doradus. We have three optical spectra at hand, two archival HST spectra (Hα and 3200 − 4800 Å) and one spectrum obtained by Schnurr et al. (2008). Considerable differences can be detected between these data. For example, the HST spectrum exhibits significantly higher emission-line strengths of the He iiλ 4686 and the N ivλ 4060 lines. These observational discrepancies might result from a nearby source that contaminated these observations. Due to the higher spatial resolution of the HST compared to the ground-based telescopes, we rely primarily on HST data.

The optical narrowband magnitude listed in the BAT99 catalog is in excellent agreement with the HST spectrum, which covers the spectral range from 3200 Å to 4800 Å. Another HST spectrum covering Hα, on the other hand, exhibits a flux of approximately 0.1 dex higher. Thus, the uncertainty in the derived luminosity (Table 2) is higher than for the other stars in our sample.

For the infrared part of the SED, we used the photometric data obtained by Kato et al. (2007), instead of the low quality 2MASS data. With the exception of the J-band magnitude, the photometric data measured by Kato et al. (2007) appear to be unaffected by nearby sources. However, in comparison to the optical data, we find an infrared excess that might be caused by nearby sources, a hidden companion, or dust emission.

BAT99 105

is listed as a transition-type star in the BAT99 catalog, but has been demoted to O2 If* by Crowther & Walborn (2011). We have UV as well as optical spectra at hand for this star. The optical observations (an archival HST spectrum and a ground-based spectrum observed by Schnurr et al. 2008) possibly show two different objects. For example, the HST spectrum exhibits the He iiλ 4201,4542 lines in absorption, whereas these lines are in emission in the spectrum obtained by Schnurr et al. (2008). Moreover, the He iiλ 4686 and N iiiλ 4640 lines are appreciably stronger in the latter. Since the HST spectrum exhibits the same appearance as the UVES spectrum shown by Crowther & Walborn (2011), we choose to rely on the HST data in the optical spectral range.

BAT99 105 is suspected to be a binary due to the X-ray emission detected by Guerrero & Chu (2008a), although no significant radial velocity variations were detected by Schnurr et al. (2008). We note that the optical HST and the IUE short-wavelength spectrum can be consistently reproduced by the same single-star model.

This object was intensively analyzed by Heap et al. (1991), Pauldrach et al. (1994), de Koter et al. (1997), and Doran & Crowther (2011). On the basis of modern stellar atmosphere models, Doran & Crowther (2011) derived a stellar temperature of T = 49.8 kK, which agrees with our own results. In comparison to the latest comprehensive analysis by de Koter et al. (1997), our fit results in a 5 kK higher stellar temperature, an identical luminosity, but a considerably lower mass-loss rate (a factor of 3.6 lower). Pauldrach et al. (1994) have derived a mass-loss rate of similar extent as de Koter et al. (1997). However, a mass-loss rate as high as derived by Pauldrach et al. (1994) and de Koter et al. (1997) results in a considerable overprediction of the emission lines. The studies of Pauldrach et al. (1994) and de Koter et al. (1997) were based on UV spectra alone.

BAT99 106

is a WN5h star (BAT99) located in the core of R 136. This star was studied by de Koter et al. (1997), Crowther & Dessart (1998), and Crowther et al. (2010). Crowther et al. (2010) report it to be one of the most massive stars known so far. Our independent analysis basically confirms the physical parameters derived by Crowther et al. (2010).

X-ray emission was detected by Guerrero & Chu (2008a) for BAT99 106, 108, 109, and 110 with the Chandra satellite. Since these stars in the tight cluster R136 cannot be resolved by this instrument, we treat BAT99 106 as a single star, although the X-ray emission might be associated with it and indicate colliding winds in a binary system.

We note that in addition to the photometry given in Sect. 2.3, we used b-band photometry (Crowther & Dessart 1998) and KS-band photometry (Crowther et al. 2010) in the SED fit.

BAT99 107

has been identified by Taylor et al. (2011) as a massive SB2 binary system consisting of two O-type stars. Moffat (1989) found a radial velocity variation with a period of 52.7 d. However, Schnurr et al. (2008) could not confirm this period.

BAT99 108

is the most massive star in the core of R 136 (Crowther et al. 2010). It is listed as WN5h star in the BAT99 catalog and has been analyzed by de Koter et al. (1997), Crowther & Dessart (1998), and Crowther et al. (2010). In comparison to the most recent analysis by Crowther et al. (2010), we obtained fairly similar stellar parameters. Note that our SED fit (Fig. C.43) matches the HST spectra (UV and optical) consistently with the KS-band photometry (Crowther et al. 2010). We ignore the optical photometry (BAT99, Crowther & Dessart 1998), which is inconsistent with the calibrated HST spectrum. X-ray emission is associated with BAT99 108 (cf. comment on BAT99 106).

BAT99 109

is another WN5h star (BAT99) in the core of R 136, previously analyzed by Crowther & Dessart (1998) and Crowther et al. (2010). We have UV and optical HST spectra for this object. According to de Koter et al. (1997), the HST spectra of BAT99 109 are contaminated by the flux of BAT99 108 situated only 0.1″ away. Since the optical HST spectrum is less affected by this contamination (de Koter et al. 1997), we primarily rely on this spectrum.

This contamination is probably the reason that our SED fit cannot simultaneously reproduce the continuum slope of the UV and optical HST spectrum. However, the optical spectrum can be matched in conformity with KS-band photometry (Crowther et al. 2010). Nevertheless, the luminosity of this star is subject to a large uncertainty, since the optical HST spectrum of BAT99 109 is also contaminated to a certain extent. Despite these uncertainties, we obtained nearly the same stellar parameters as previously derived by Crowther et al. (2010). With a luminosity of log  (L/L) = 6.69, it is one of the most luminous objects in our sample. We point out that a fit of the photometry (Crowther & Dessart 1998, BAT99; Crowther et al. 2010) alone does not result in a lower luminosity. Note that X-ray emission is associated with BAT99 109 (cf. comment on BAT99 106).

BAT99 110

was classified as O2 If* by Crowther & Walborn (2011). The preceding studies by Heap et al. (1994) and de Koter et al. (1997) report nearly equal physical parameters. In contrast to these studies, our best fitting model has a stellar temperature of T = 50 kK which is 7.5 kK higher. Lower temperatures are excluded by our analysis, since, relative to the observed line strengths, our grid models with lower stellar temperatures overestimate the N iii lines and underestimates the N iv and N v lines. This temperature discrepancy probably arises due to the inclusion of line blanketing in our models, which was not accounted for in the stellar atmosphere models at the time of the earlier studies. Moreover, our new study results in a luminosity which is factor of two higher, while the mass-loss rate is about 40% lower compared to the results obtained by Heap et al. (1994) and de Koter et al. (1997). We note that it is not possible to reproduce the UV and optical spectrum with the same reddening parameters. A satisfying fit of the UV spectra can only be achieved with a color excess of Ebv = 0.1 and a luminosity of log  (L/L) = 5.33. However, these values are considerably low compared to the results (see Table 2) derived from the photometry (Crowther & Dessart 1998; Breysacher et al. 1999) and the optical HST spectrum. Note that X-ray emission is associated with BAT99 110 (cf. comment on BAT99 106).

BAT99 111

is a WN9ha star (BAT99) in the center of R 136. Although Schnurr et al. (2009a) could not find radial velocity variations for this object, we treat it as a binary suspect because of the X-ray emission reported by Townsley et al. (2006). We have two flux-calibrated HST spectra at hand, but we note that the model cannot perfectly reproduce the continuum shape of the UV and the optical spectra with the same reddening parameters. However, the luminosity derived from the UV spectrum is only 0.06 dex lower than the value (Table 2) derived from the optical HST spectrum and optical photometry (BAT99). Due to a hydrogen mass-fraction of XH = 0.7, this object appears not to be in an advanced evolution stage. Therefore, we disagree with the conclusion of Schnurr et al. (2009a) that this star is more evolved than the other stars in the core of R 136.

BAT99 112

is another WN5h star in the core of R 136 that is a candidate for a long-period binary system (Schnurr et al. 2009a). An indirect argument in favor of the binary status is the hard X-ray emission detected by Townsley et al. (2006) and Guerrero & Chu (2008a). We expect that the potential companion does not contribute much to the bolometric luminosity, since no spectral lines of the potential companion can be recognized in the HST spectrum. Thus, we analyzed this star as a single star, despite its pending binary status.

In the SED fit, the slope of the calibrated HST spectrum and the optical photometry (BAT99) can be consistently reproduced with the same luminosity and color excess. In contrast, the -band photometry (Crowther et al. 2010) exhibits a clear excess, which might be caused by the potential companion or dust emission. We note that the available HST spectrum does not cover the Hα and Hβ lines, so that the hydrogen abundance is derived using the higher members of the Balmer series alone. Since these lines are rather weak, the hydrogen abundance is subject to a relatively high uncertainty. We estimate a hydrogen mass-fraction of XH = 0.2, which is 0.1 dex lower than previously derived by Crowther et al. (2010). The temperature obtained by these authors is slightly lower, while the mass-loss rate and the luminosity is more than 0.2 dex higher compared to the results of the present paper.

BAT99 113

is a transition type O2 If*/WN5 star (Crowther & Walborn 2011; Evans et al. 2011) located close to the core of 30 Doradus. We have two optical spectra at hand, an archival HST spectrum and a ground-based spectrum taken by Schnurr et al. (2008). These spectra, however, clearly deviate from each other. For example, the equivalent width of the He iiλ 4686 line differ by roughly a factor of two. Although the S/N is lower in the HST spectrum, we primarily use this spectrum in our analysis because of the distinctive spatial resolution of the HST. The star was identified by Schnurr et al. (2008) as a binary system with a period of 4.7 d. Since no indications of a companion were found in the spectra, we expect that the flux contribution of the companion is insignificant for the analysis of the WN star. Unfortunately, the HST spectra do not cover the Hα and Hβ lines. Consequently, the hydrogen abundance given in Table 2 is derived from weak H γ and H δ lines alone.

BAT99 114

is another transition type O2 If*/WN5 star (Crowther & Walborn 2011; Evans et al. 2011) in the vicinity of the 30 Doradus core. We have two optical spectra at hand, an archival HST spectrum and a spectrum obtained by Schnurr et al. (2008). Since the latter lacks a sufficient subtraction of the diffuse background, our analysis is mainly based on the HST spectrum. However, this spectrum covers only the wavelength range from 3300 to 4800 Å, where merely the H γ and H δ lines can be found as indicators for the hydrogen abundance. Due to the X-ray emission detected by Guerrero & Chu (2008a) we consider this object as a binary suspect, although Schnurr et al. (2008) could not find periodic radial velocity variations.

BAT99 116

was classified as WN5h:a by Schnurr et al. (2008). These authors have reported radial velocity variations, but found no periodicity. Schnurr et al. (2009a) noted that this object is likely a long periodic binary system, in agreement with the strong X-ray emission detected by Guerrero & Chu (2008a,b). Thus, we consider this object as a binary suspect. Unfortunately, the only spectrum at hand (Schnurr et al. 2008) lacks a subtraction of the diffuse background. Therefore, we are not able to establish a robust estimate of the hydrogen abundance.

BAT99 117

is a WN5ha star (Foellmi et al. 2003b) located in the northern part of 30 Doradus. The stellar temperature and luminosity derived in our analysis are moderately lower compared to the results obtained by HK2000, whereas the mass-loss rate is 50% higher. We also derive a 0.1 dex higher hydrogen abundance. However, our analysis suffers from an insufficient subtraction of the diffuse background in the spectrum from Foellmi et al. (2003b). This spectrum exhibits strong absorption lines in place of the O iii-nebular emission lines at 4959 Å and 5007 Å, which might be caused by an overcorrection of the background. If this is true, the Balmer series will probably be impaired by the inadequate nebular subtraction as well. In this case, the hydrogen abundance listed in Table 2 is a subject to high uncertainty.

By comparing the HRD position to stellar evolution models performed by Meynet & Maeder (2005), we derive an initial mass of roughly Minit = 120 M. Thus, this star belongs to the category of very massive stars. Foellmi et al. (2003b) did not find periodic radial velocity variations.

BAT99 118

is a WN6h star (BAT99), which is treated as a binary candidate in the radial velocity study by Schnurr et al. (2008). An indirect argument in favor of the binary status is the strong and hard X-ray flux detected by Guerrero & Chu (2008a). New X-shooter observations performed by Sana et al. (2013b) revealed it to be a SB2 binary with a mass ratio close to unity. Thus, the system consists of two similar WN stars, which were classified as WN5–6h + WN6–7h by Sana et al. (2013b). We analyzed this system as if it were a single star.

Our best fit is achieved at a stellar temperature of T = 47 kK, which is only marginally higher than the value recently derived by Doran et al. (2013). The mass-loss rate obtained by these authors agrees well with the value presented in this work. In contrast, Crowther & Dessart (1998) obtained a mass-loss rate that is a factor of two lower, while the stellar temperature derived by these authors (on the basis of unblanketed model atmospheres) is 10 kK lower. Crowther & Dessart (1998) and Doran et al. (2013) obtain luminosities that are lower compared to the value (log  (L/L) = 6.66) derived in this work. Crowther & Dessart (1998) obtained a value that is 0.32 dex lower, whereas the analysis carried out by Doran et al. (2013) results in a luminosity that is 0.25 dex lower. As opposed to this, the luminosity estimate by Sana et al. (2013b) results in a luminosity that is 0.14 dex higher compared to our new results.

These differences need to be considered, if the luminosity is used to derive the current mass of the stellar content. On the basis of their high luminosity, Sana et al. (2013b) derived a current mass between 80 M and 205 M for each component in BAT99 118. For the initial masses, we obtained about Minit = 100 M for each WN component by comparing the empirical HRD position of this object (see Figs. 7 and 10) with the stellar evolution tracks by Meynet & Maeder (2005) and Yusof et al. (2013).

A description of the FUSE spectra (not considered in this work) can be found in Willis et al. (2004). The authors derived a terminal velocity of v = 1847 km s-1 which is about 250 km s-1 higher than the value used for the calculation of our grid models.

BAT99 119

is a WN6h star, which is listed as a single-line spectroscopic binary (SB1) in the BAT99 catalog with a period of 25.2 d (Moffat 1989). In contrast, Schnurr et al. (2008, 2009b) find a period of 158.8 d, combining their radial velocity data with that of Moffat (1989) and new polarimetric data. According to Schnurr et al. (2009b), the companion is most likely an O-type star, although no obvious trace of the companion can be found in the spectrum. With the exception of a slightly smaller emission-line strength, the spectrum of BAT99 119 resembles that of BAT99 118, which is a SB2 of two similar WN stars (Sana et al. 2013b). Considering that the resolving power of our optical spectra is only R ≈ 1000, we also stress the possibility of a binary system encompassing two WN stars with a mass ratio close to unity.

Assuming an O-type companion, Schnurr et al. (2009b) were able to give constraints on the properties of both components in this binary system. They found the WN star to be the considerably more luminous component. Thus, we estimate the flux contribution of the companion to be negligible in the UV and optical spectral range, which is in accordance with the moderate infrared excess found in our analysis.

Similar to BAT99 118, we derive a stellar temperature of T = 47 kK, which is about 15 kK higher than that obtained by Crowther & Smith (1997) on the basis of unblanketed model atmospheres. In comparison to this former study, our substantially higher temperature entails a luminosity increase by a factor of roughly 2.5 (log  (L/L) = 6.57). The mass-loss rate derived here, on the other hand, is nearly identical to the value given by Crowther & Smith (1997), whereas Schnurr et al. (2009b) estimated a mass-loss rate on the basis of their polarimetric data, which is a factor of two higher.

The latter authors derived a dynamical mass of Mdyn = 116±33 M for the WN component. The initial mass obtained from the HRD position (Figs. 7 and 10) will be approximately Minit = 150 M, if the WN star contributes most to the overall flux of the binary system (WN + OB).

BAT99 120

is classified as WN9h star (BAT99) and may be a dormant LBV, according to Crowther et al. (1995a). We have two optical spectra at hand, an archival AAT spectrum (see Sect. 2.3) and a coadded spectrum observed by Foellmi et al. (2003b). The stellar parameters presented in Table 2 rely mainly on the latter spectrum, due to their high S/N. This spectrum is best reproduced by a model with a stellar temperature of T = 32 kK, while a model with T = 35 kK is more appropriate for the AAT spectrum. BAT99 120 was previously analyzed by Pasquali et al. (1997) and Crowther et al. (1995a). They obtained stellar temperatures of T = 38.9 kK and T = 30 kK, respectively. However, stellar temperatures higher than T = 35 kK would spoil the fit of the He i and He ii lines in both optical spectra.

In addition to the optical spectra, we used flux-calibrated UV spectra, which were obtained with the HST and the IUE satellite. Fitting the continua of these spectra and the available photometric data (2MASS and optical photometry from the BAT99 catalog), the luminosity is found to be log  (L/L) = 5.58, while the color excess amounts to Ebv = 0.15. In contrast, the luminosities presented by Crowther et al. (1995a) and Pasquali et al. (1997) are higher, while the derived color excess agrees with our study. The luminosity derived by Crowther et al. (1995a) is only slightly higher, whereas Pasquali et al. (1997) derived a luminosity of a factor of 2.5 higher. This deviation can be attributed the higher temperature and thus higher bolometric correction derived by Pasquali et al. (1997). In comparison to the former studies, the mass-loss rate is slightly lower in our new study.

BAT99 122

had never been analyzed by means of model atmospheres before. The infrared excess reported by Hyland et al. (1978) can be seen in our fit of the SED (Fig. C.49) as well. The star was classified as WN5h by Evans et al. (2011).

BAT99 124

belongs to the WN4 subclass (Foellmi et al. 2003b). It is analyzed by means of model atmospheres for the first time in this work. The uncertainty of the obtained hydrogen abundance is large, since the available spectrum is strongly contaminated with nebular emission, which is evident by the strong O iiiλλ 4959,5007 nebular emission lines. According to Martín-Hernández et al. (2005), the shell structure of NGC 2077 may be caused by the feedback of BAT99 124.

BAT99 126

is listed in the BAT99 catalog as a WN3+O7 binary candidate. Foellmi et al. (2003b) found a period of 25.5 d, but noted that more data are needed to verify this result. Therefore, we treat this object as a binary suspect, although it is likely a binary. The X-ray emission detected by Guerrero & Chu (2008a) is further indirect evidence for the binary status. Foellmi et al. (2003b) reclassified the companion to O8 and the WN component to WN4b. The luminosity of this object is derived from photometry only, since no flux-calibrated spectra are available.

BAT99 128,

classified as WN3b (Foellmi et al. 2003b), is a typical WN3 star. An observational discrepancy exists for this star between the photometry obtained by Crowther & Hadfield (2006) and the flux-calibrated spectrum obtained by Torres-Dodgen & Massey (1988). The noisy spectrum measured by Torres-Dodgen & Massey (1988) exhibits a higher flux, which results in a SED fit of only moderate quality and an unreliably low color excess of Ebv = 0.01 mag. The spectrophotometry from Crowther & Hadfield (2006) fits much better to the 2MASS and the IRAC photometry and results in a SED fit of higher quality and a more convincing color excess of Ebv = 0.17 mag. Foellmi et al. (2003b) found a radial velocity that is significantly below the mean vrad of their sample, suggesting that this star might be a runaway. We note that BAT99 128 falls into the regime of parameter degeneracy (cf. Sect. 4.2).

BAT99 129

is an eclipsing binary with a WN3(h)a star (BAT99) as primary component and an O5V companion (Foellmi et al. 2006). Foellmi et al. (2003b) find a radial velocity period of 2.76 d, but no X-ray emission was detected by the Rosat satellite (Guerrero & Chu 2008b). We estimate a hydrogen mass-fraction of XH = 0.2, thus confirming the above classification. Foellmi et al. (2006) derived a luminosity of log  (L/L) = 4.97 for the WR component, which would make this the faintest WN star known in the LMC (cf. Fig. 7). This result cannot be confirmed by our analysis. Assuming that the luminosity derived in our analysis is valid for the whole system, and applying the luminosity ratio of 0.3 derived by Foellmi et al. (2006), we obtain a luminosity of log  (L/L) = 5.68 for the WN component. This luminosity is considerably higher than the value derived by Foellmi et al. (2006) and at the upper end of the luminosity range derived for the other presumably single WN3 stars in the LMC.

BAT99 130

is the second WN11h star (BAT99) in our sample. Our new analysis confirms the stellar parameters derived in the former study by Crowther & Smith (1997) with the exception of a significantly lower hydrogen abundance.

BAT99 131

was classified as WN4b (Foellmi et al. 2003b) and had not been analyzed before. The available IUE spectra are not uniform and do not exhibit any prominent emission line. Therefore, we reject these spectra from our analysis, although the flux of the IUE long-wavelength spectrum is compatible to the available photometry. Thus, the luminosity obtained in our analysis is derived from visual narrowband, 2MASS and IRAC photometry alone.

BAT99 132

is a WN4b(h) star (Foellmi et al. 2003b), analyzed for the first time in this paper. The best fit of the spectra is achieved with a hydrogen-free model, although the presence of residual hydrogen in the stellar atmosphere of this object was reported by Foellmi et al. (2003b).

BAT99 133

is the third WN11h (BAT99) in the LMC and one of the only three WN stars detected at 24 μm with the IRAC instrument aboard the Spitzer space telescope (Bonanos et al. 2009). According to Humphreys & Davidson (1994) and Weis (2003), this star is suspected to be an LBV in its quiescent phase. Likewise, Walborn (1982) and Bonanos et al. (2009) have noted the spectroscopic similarities between BAT99 133 and the LBV BAT99 83 in its minimum. Contrary to this, our spectra of these two stars exhibit clear differences. For example, the He iiλ 4686 line is absent in the spectrum of BAT99 83, whereas a relatively small emission line is present in the spectrum of BAT99 133. Further different features are the He i lines, which are much more prominent in the spectrum of BAT99 133.

BAT99 133 was previously analyzed by Crowther & Smith (1997) and Pasquali et al. (1997). The former obtained a stellar temperature of T = 28.3 kK, which is confirmed by our analysis. Pasquali et al. (1997), on the other hand, derived a stellar temperature of roughly 8 kK higher. However, our models clearly underpredict the observed He i/He ii ratio at this higher temperature. In principle, the same applies to the mass-loss rate and luminosity, where we can confirm the results obtained by Crowther & Smith (1997). Contrary to this, Pasquali et al. (1997) derived values for the mass-loss rate and the luminosity that are roughly twice as high.

A study of the nebula associated with BAT99 133 can be found in Pasquali et al. (1999) and Weis (2003).

BAT99 134

is listed as WN4b star in the BAT99 catalog. In this first spectroscopic analysis with stellar atmosphere models, we derived physical parameters typical for the WN4 subclass.

Dopita et al. (1994) discovered a ring nebula that surrounds BAT99 134. As no He ii nebular emission is detected by (Nazé et al. 2003b), they obtain an upper limit for the number of He ii ionizing photons delivered by the exciting star, which amounts to <3.2 ×1045 He ii ionizing photons per second. This agrees with our final model, which does not produce a significant number of He ii ionizing photons (see Table A.3).

Appendix C: Spectral fits

In this section, we present the spectral fits of all stars analyzed in this study. The individual plots encompass the fit of the spectral energy distribution (top panel) to the photometric and flux-calibrated spectra as well as the fits to the normalized optical and UV spectra (lower panels), when available. The observations are plotted in blue, whereas the synthetic spectrum of the best-fitting model shown in red.

Some of our stellar atmosphere models with stellar temperatures below T = 32 kK exhibit spurious emission lines in the spectral range from about 1900 Å to 2100 Å. These emission features, which are not observed, originates from the third ionization stage of our generic model atom representing the iron-group elements. We note that the presence of these emission features is only a cosmetic issue and has no impact on the derived stellar parameters.

thumbnail Fig. C.1

Spectral fit for BAT99 001 and BAT99 002.

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thumbnail Fig. C.2

Spectral fit for BAT99 003 and BAT99 005.

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thumbnail Fig. C.3

Spectral fit for BAT99 006 and BAT99 007.

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thumbnail Fig. C.4

Spectral fit for BAT99 012 and BAT99 013.

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thumbnail Fig. C.5

Spectral fit for BAT99 014 and BAT99 015.

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thumbnail Fig. C.6

Spectral fit for BAT99 016 and BAT99 017.

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thumbnail Fig. C.7

Spectral fit for BAT99 018 and BAT99 019.

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thumbnail Fig. C.8

Spectral fit for BAT99 021 and BAT99 022.

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thumbnail Fig. C.9

Spectral fit for BAT99 023 and BAT99 024.

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thumbnail Fig. C.10

Spectral fit for BAT99 025 and BAT99 026.

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thumbnail Fig. C.11

Spectral fit for BAT99 027 and BAT99 029.

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thumbnail Fig. C.12

Spectral fit for BAT99 030 and BAT99 031.

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thumbnail Fig. C.13

Spectral fit for BAT99 032 and BAT99 033.

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thumbnail Fig. C.14

Spectral fit for BAT99 035 and BAT99 036.

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thumbnail Fig. C.15

Spectral fit for BAT99 037 and BAT99 040.

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thumbnail Fig. C.16

Spectral fit for BAT99 041 and BAT99 042.

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thumbnail Fig. C.17

Spectral fit for BAT99 043 and BAT99 044.

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thumbnail Fig. C.18

Spectral fit for BAT99 046 and BAT99 047.

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thumbnail Fig. C.19

Spectral fit for BAT99 048 and BAT99 049.

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thumbnail Fig. C.20

Spectral fit for BAT99 050 and BAT99 051.

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thumbnail Fig. C.21

Spectral fit for BAT99 054 and BAT99 055.

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thumbnail Fig. C.22

Spectral fit for BAT99 056 and BAT99 057.

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thumbnail Fig. C.23

Spectral fit for BAT99 058 and BAT99 059.

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thumbnail Fig. C.24

Spectral fit for BAT99 060 and BAT99 062.

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thumbnail Fig. C.25

Spectral fit for BAT99 063 and BAT99 064.

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thumbnail Fig. C.26

Spectral fit for BAT99 065 and BAT99 066.

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thumbnail Fig. C.27

Spectral fit for BAT99 067 and BAT99 068.

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thumbnail Fig. C.28

Spectral fit for BAT99 071 and BAT99 072.

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thumbnail Fig. C.29

Spectral fit for BAT99 073 and BAT99 074.

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thumbnail Fig. C.30

Spectral fit for BAT99 075 and BAT99 076.

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thumbnail Fig. C.31

Spectral fit for BAT99 077 and BAT99 078.

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thumbnail Fig. C.32

Spectral fit for BAT99 079 and BAT99 080.

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thumbnail Fig. C.33

Spectral fit for BAT99 081 and BAT99 082.

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thumbnail Fig. C.34

Spectral fit for BAT99 086 and BAT99 088.

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thumbnail Fig. C.35

Spectral fit for BAT99 089 and BAT99 091.

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thumbnail Fig. C.36

Spectral fit for BAT99 092 and BAT99 093.

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thumbnail Fig. C.37

Spectral fit for BAT99 094 and BAT99 095.

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thumbnail Fig. C.38

Spectral fit for BAT99 096 and BAT99 097.

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thumbnail Fig. C.39

Spectral fit for BAT99 098 and BAT99 099.

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thumbnail Fig. C.40

Spectral fit for BAT99 100 and BAT99 102.

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thumbnail Fig. C.41

Spectral fit for BAT99 103 and BAT99 104.

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thumbnail Fig. C.42

Spectral fit for BAT99 105 and BAT99 106.

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thumbnail Fig. C.43

Spectral fit for BAT99 107 and BAT99 108.

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thumbnail Fig. C.44

Spectral fit for BAT99 109 and BAT99 110.

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thumbnail Fig. C.45

Spectral fit for BAT99 111 and BAT99 112.

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thumbnail Fig. C.46

Spectral fit for BAT99 113 and BAT99 114.

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thumbnail Fig. C.47

Spectral fit for BAT99 116 and BAT99 117.

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thumbnail Fig. C.48

Spectral fit for BAT99 118 and BAT99 119.

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thumbnail Fig. C.49

Spectral fit for BAT99 120 and BAT99 122.

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thumbnail Fig. C.50

Spectral fit for BAT99 124 and BAT99 126.

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thumbnail Fig. C.51

Spectral fit for BAT99 128 and BAT99 129.

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thumbnail Fig. C.52

Spectral fit for BAT99 130 and BAT99 131.

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thumbnail Fig. C.53

Spectral fit for BAT99 132 and BAT99 133.

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thumbnail Fig. C.54

Spectral fit for BAT99 134.

Open with DEXTER


© ESO, 2014

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