Volume 581, September 2015
|Number of page(s)||30|
|Published online||26 August 2015|
Atomic model used in the stellar atmosphere calculations.
Spectra used in the analysis of the putatively single WN stars in the SMC.
Abundances derived in the analysis of the putatively single WN stars in the SMC (mass fraction in percent).
Number of ionizing photons and Zanstra temperatures for the putatively single WN stars in the SMC.
(alias: AzV 2a, SMC-WR1): we achieved the best fit with a stellar temperature of 79.4 kK, while Martins et al. (2009) derived a value of 65.4 kK. At this low temperature, however, all observed N v lines are considerably underpredicted by our models. In addition, the synthetic spectra of these models posses a distinct N iv λ 4060 emission line that is not observed for this object. The N iv emission in the model spectra already vanishes at approximately 70 kK. Because of a better fit of the N v lines we favor, however, the model with a slightly higher temperature. Another indication of the higher temperature is the C iv λ 1551 line, which also vanishes at approximately 70 kK and which is indeed not observed for this object. The higher temperature in comparison to the findings by Martins et al. (2009) also results in a stellar luminosity which is 0.32 dex higher. Although both studies are based on the same ESO VLT UVES observations, the reduced data have some minor differences. Furthermore, the differences in the stellar parameters are partly due to the fact that the models do not reproduce all lines in the observed spectrum consistently. Hence the derived stellar parameters depend on which lines are considered to be most significant for the line fit. We note that the mass-loss rate is almost identical in both studies.
(alias: AzV 39a, SMC-WR2): Testor (2001) found this star to be embedded in the H ii region N 28. The stellar parameters of SMC AB 2 were already studied by Martins et al. (2009). Within the expected error margins, the stellar temperature and the luminosity obtained in this study are compatible with the findings of the previous authors. Martins et al. (2009) assume a clumping factor of D = 10, while our fit points to a lower value of about four. Taking this into account, the mass-loss rates obtained in both studies deviate by only 0.1 dex, which is within the expected error range as well.
(alias: AzV 81, Sk 41, SMC-WR4): it is the only WN6h star in the SMC which appears to be single (Foellmi et al. 2003). It was analyzed before by Crowther (2000) and Martins et al. (2009). Compared to the findings of these two studies, our analysis results in a slightly higher stellar temperature (by approximately 2 − 3 kK) and a mass-loss rate that is lower by about 0.1 dex when the different clumping factors are taken into account. The luminosity deduced from the SED fit lies within the range determined by the previous analyses (Crowther 2000; Martins et al. 2009), while the deviation amounts to approximately 0.1 dex in both cases. We also note that the hydrogen abundance deduced in our analysis is lower by 5% and 10% in relation to the results of Crowther (2000) and Martins et al. (2009), respectively.
(alias: MVD 1, SMC-WR9): this object belongs to the hottest stars in the sample. The temperature determination is base primarily on the antagonistic behavior of the nitrogen lines and the helium lines (especially He ii λ 5412). While the nitrogen lines tend to favor lower temperatures, a superior fit of the He ii λ 5412 line requires higher model temperatures. We note that a weak O iii λ 5007 nebular line is visible in the optical spectrum, which could indicate an associated nebula.
Unfortunately, the quality of the fit in the FUSE range is poor, since the continuum shape of the FUSE spectra cannot be reproduced by the models. This seems to be an issue of the observation rather than a model deficiency. The observed flux shortward of 1000 Å is approximately a factor of two lower than predicted by the model. Since no significant ionization edge is expected in this spectral region, we assume that the observation at hand is affected by inaccurate flux in at least one of the two SiC channels (SiC refers to the silicon carbide coating of the corresponding mirror) of the FUSE satellite. This is consistent with a bump in the spectrum at about 1085 Å, where a gap between the segments of the LiF channels (LiF refers to the lithium fluoride coating of the corresponding mirror) is covered by the SiC channels.
(alias: SMC-WR10): the fit of the FUSE spectral range suffers from the same caveats as described for SMC AB 9 (see corresponding comment). In contrast to the other objects in our sample, the best fitting model assumes a velocity field in the form of a double-β law (Hillier & Miller 1999) as defined by Todt et al. (2015) with β1 = 1 and β2 = 4. For SMC AB 10, the choice of a double-β law is mainly motivated the exceptional line profile of the He ii λ 4686 line that has a broad base and a very narrow line peak. With respect to a model using a single-β law with β = 1, the mass-loss rate is increased by about 0.1 dex and the terminal wind velocity is higher by about 600 km s-1.
(alias: SMC-WR11): a bright red object is located about 1.2″ from SMC AB 11 (see also Crowther & Hadfield 2006). Since the spectra at hand (Foellmi et al. 2003) have a spatial resolution on the order of 1″, we note the possibility of a contamination. Figure B.1 shows two images taken with the Wide-Field Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope (HST) in the F300W and F606W filter, respectively. The position of SMC AB 11 is marked with a red circle. The HST observations clearly show that the WR star dominates the flux in the UV, while both objects exhibit approximately the same flux level in the F606W filter. In the IR on the other hand, the flux is dominated by the non-WR object.
HST-WFPC2 images of the region within about 8″ distance of SMC AB 11. Left panel: image taken in the F606W filter. Right panel: image taken in the F300W filter. The red circle marks the position of SMC AB 11. The images were retrieved from the HST archive. North is up and east is left.
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The first panel of Fig. C.6 shows the SED fit derived in our analysis. Since no flux-calibrated spectra are available for this object, the luminosity are derived solely from photometry. A variety of photometric measurements is available within a radius of 2″ from the coordinates given for SMC AB 11 in the SIMBAD database. In comparison to the line-of-sight companion, SMC AB 11 is located farther to the north (see Fig. B.1). Since the measurements performed by Zaritsky et al. (2002) and Kato et al. (2007) were obtained at a similar position, we mainly used these values as well as the spectrophotometry obtained by Crowther & Hadfield (2006) to constrain the luminosity of SMC AB 11. In Fig. C.6, the corresponding photometry marks and the model SED for SMC AB 11 are shown by brown boxes and a brown dotted line, respectively. Other photometry (Zaritsky et al. 2002; Monet et al. 2003; Oey et al. 2004; Gordon et al. 2011; Cutri et al. 2012a,b), which presumably incorporates contributions from both objects, were used to construct the observed SED that combines both objects (blue boxes in Fig. C.6). In this figure, the red straight line represents the model that incorporates both the WN star as well as flux contribution from the non-WR object, which is assumed to be a black body (green dashed line). Because of the flux contribution of the line-of-sight companion, the uncertainty in the derived luminosity of SMC AB 11 is higher than for the other stars in our sample. In the lower panels of Fig. C.6, the relative contribution of the WN star to the combined spectrum (red dashed line) is shown by the brown dotted line.
Foellmi et al. (2003) mention the presence of N iv λ 4060 emission in an ESO-NTT spectrum of SMC AB 11,
although neither the spectrum at hand nor the spectrum published by Massey & Duffy (2001) exhibits this line. We note that stellar atmosphere models with temperatures of about 65 kK would be necessary to reproduce this line simultaneously with the N v emission lines. This would be considerably lower than the 89 kK derived in this analysis.
(alias: SMC-WR12): it is the object with the highest surface temperature in our sample. In the same way as the other WN3 stars only one ionization stage per element is visible in our spectra of SMC AB 12, so that the temperature determination has to rely on the N v lines and the He ii lines. At least 112 kK are necessary to reproduce the He ii λ 5412 line. Moreover at these temperatures the model spectra do not show He ii lines originating from the fifth quantum level, which is consistent with the observations.
The following figures show the final spectral fit of each object analyzed in this paper. In all plots the upper panel shows the fit of the SED, while the lower panels exhibit the fit of the normalized optical and UV spectrum, if available. The best-fitting model is plotted in red, while all observations are shown in blue.
The spectral fit for SMC AB 11. The brown boxes in the top panel are the photometry attributed to the WR star alone, while the blue boxes refer to photometry that comprises the flux from a companion separated by approximately 1.2″ (see Appendix B for details). The brown dotted line represents the best fitting PoWR model for the WN star, while the green dashed line shows a black body spectrum at 4.6 kK as a rough approximation to the nearby red star. The photometry with large apertures (blue boxes) are obviously determined by the red star for λ ≳ 8000 Å. The straight red line is the combined model SED that incorporates the flux from the WN star and the line-of-sight companion.
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© ESO, 2015
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