Free Access
Volume 543, July 2012
Article Number A95
Number of page(s) 42
Section Stellar atmospheres
Published online 04 July 2012

Online material

Appendix A: Comparison with CMFGEN

Figures A.1 to A.8 provide a detailed comparison between H/He and N iii/N iv/N vcmfgen spectra for models d2v, d4v, s2a, and s4a (see Table 3), and corresponding fastwind profiles from closest or almost closest grid models, for a nitrogen abundance of [N] = 8.78 and [N] = 7.78. If not explicitly stated else, no convolution has been applied to the spectra. For details, see Sect. 5.

thumbnail Fig. A.1

Model d4v at [N] = 8.78. Comparison of H/He/N spectra from cmfgen (green) and fastwind, at the closest grid-model (black: Teff = 41 kK, log g = 4.0, log Q =  −12.8, [N] = 8.78) and at neighboring grid models with Teff = 40 kK (red) and Teff = 42 kK (blue). To allow for an easier comparison, all profiles have been convolved with vsini = 30 km   s-1.

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thumbnail Fig. A.2

Model d4v at [N] = 7.78 (solar). Comparison of N spectra from cmfgen (green) and fastwind, at the closest grid-model. (black: Teff = 41 kK, log g = 4.0, log Q =  −12.8, [N] = 7.78). The H/He spectra remain as in Fig. A.1. No convolution applied.

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thumbnail Fig. A.3

Model d2v at [N] = 7.78 (solar). Comparison of N spectra from cmfgen (green) and fastwind, at the closest grid-model (black: Teff = 46 kK, log g = 4.0, log Q =  −12.45, [N] = 7.78), and at the neighboring grid model with Teff = 47 kK (red). The H/He spectra remain as in Fig. A.4.

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thumbnail Fig. A.4

Model d2v at [N] = 8.78. Comparison of H/He/N spectra from cmfgen (green) and fastwind, at the closest grid-model (black: Teff = 46 kK, log g = 4.0, log Q =  −12.45, [N] = 8.78) and at neighboring grid model with Teff = 47 kK (red). No convolution has been applied.

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thumbnail Fig. A.5

Model s4a at [N] = 8.78. Comparison of H/He/N spectra from cmfgen (green) and fastwind, at the closest grid-model (black: Teff = 39 kK, log g = 3.5, log Q =  −12.10, [N] = 8.78).

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thumbnail Fig. A.6

Model s4a at [N] = 7.78 (solar). Comparison of N spectra from cmfgen (green) and fastwind, at the closest grid-model (black: Teff = 39 kK, log g = 3.5, log Q =  −12.10, [N] = 7.78). The H/He spectra remain as in Fig. A.5.

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thumbnail Fig. A.7

Model s2a at [N] = 7.78 (solar). Comparison of N spectra from cmfgen (green) and fastwind, at the (almost) closest grid-model (black: Teff = 46 kK, log g = 3.8, log Q =  −12.10, [N] = 7.78). The H/He spectra remain as in Fig. A.8.

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thumbnail Fig. A.8

Model s2a at [N] = 8.78. Comparison of H/He/N spectra from cmfgen (green) and fastwind, at the (almost) closest grid-model (black: Teff = 46 kK, log g = 3.8, log Q =  −12.10, [N] = 8.78).

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Appendix B: Comments on the individual objects

In the following, we give specific comments on the individual objects, regarding peculiarities and problems found during our analysis. We separate between galaxy membership, and sort by luminosity class and spectral type. Line fits are displayed in Figs. B.1 to B.12, for important H/He/N lines: Hα, Hβ, Hγ, Hδ, Hϵ, He iλλ 4387, 4471, 4713, He ii(+He i)λ4026, He iiλλ4200, 4541, 4686, 6406, 6527, 6683, N iiiλλ 4003, 4097, 4195, 4379, λλ4634 − 4640 − 4642, and λλ4510 − 4514 − 4518, N ivλλ 4058, 6380, and N vλλ4603 / 4619. All spectra were corrected for radial velocity shifts.

If not explicitly stated, any comparison made in the following text refers to the results from Mas05.

B.1. LMC stars

R136-040 – O2-3.5 V

(Fig. B.1). This star could not been classified by Mas05 using the scheme by Walborn et al. (2002), because neither N iiiλλ4634 − 4640 − 4642 nor N ivλ4058 were visible in the spectra. As outlined in Sect. 6.2, for the R136 stars we have spectra from both STIS and FOS available. Taking advantage of the better quality of the STIS data and using Hα, Hγ, and He iiλ4541, we derived a similar lower limit (no He i and no nitrogen lines)! on Teff as Mas04, but a substantially larger (by 0.2 dex) gravity, which agrees better with its dwarf designation. Our analysis also resulted in a low helium content, YHe = 0.08 (Mas04: YHe = 0.1). Note the discrepancy in the cores of Hδ, Hϵ, and He iiλλ4026, 4200 when comparing with the FOS data. Such a discrepancy was also found for the remaining H/He ii lines in the FOS data, when used instead of the STIS spectra.

The missing nitrogen lines imply an upper limit for the nitrogen content corresponding to the LMC baseline abundance [N] = 6.90, which also agrees quite well with the low He content.

LH 81:W28-23 – O3.5 V((f+))

(Fig. B.2). The modeling of this star was straightforward, and we obtained similar results as Mas05. However, we considered a larger β = 1.0 to better reproduce the marginal P-Cygni profile at He iiλ4686, which might indicate a luminosity class III object (see Walborn et al. 2002). To preserve the fit of Hα, we needed to reduce . All nitrogen lines are consistent with the temperature derived from the helium ionization equilibrium. The quite large nitrogen abundance ([N] = 8.40) agrees well with the helium abundance YHe = 0.25, indicating an evolved nature of this object.

LH 101:W3-24 – O3.5 V((f+))

(Fig. B.3). We derived a somewhat cooler Teff (by 1 kK) together with a lower helium content, YHe = 0.10, which was consistent for all helium lines. Nitrogen lines are barely visible, because the spectrum of this star displays more noise than the bulk of our sample stars, caused by the use of a narrow extraction aperture to reduce effects from nebular emission for the ground-based observations (cf. Sect 6.2). Due to the noisy spectrum, we were only able to infer an upper limit for the nitrogen abundance, [N]  ≤  7.78.

LH 81:W28-5 – O4 V((f+))

(Fig. B.4). This is one of the standards used by Walborn et al. (2002) for defining the O4 V((f+)) class. A consistent analysis of the helium and nitrogen ionization equilibrium yielded a cooler temperature, Teff = 44 kK, which required a helium abundance of YHe = 0.15 to reproduce HeIλ4471. An excellent fit to most nitrogen lines from all three ionization stages was achieved for this star, indicating a significant enrichment, [N] = 8.38.

R136-018 – O3 III(f )

(Fig. B.5). Also for this O3 giant we have used data from both STIS and FOS. A consistent analysis of the H/He lines from STIS and nitrogen lines (mostly from FOS) suggested a hotter Teff, by 2 kK, as well as a higher surface gravity, by  ~ 0.1 dex. Again, we found discrepancies in the cores of the H/He ii lines from the FOS spectra, except for He iiλ4686. An acceptable fit for N iii/N iv/N v using [N] = 8.18 was possible, where only N ivλ4058  was slightly underpredicted. Even though the nitrogen lines contained in the STIS dataset, N iiiλλ4510 − 4514 − 4518 and N ivλ6380, are diluted in the continuum, they support our analysis.

LH 90:ST 2-22 – O3.5 III(f+)

(Fig. B.6). An unproblematic analysis provided the same results as obtained by Mas05, except that we opted for a lower helium abundance, YHe = 0.15. Again, a remarkable fit to the nitrogen lines was possible, at [N] = 8.58, indicating an extreme enrichment.

Sk–67 ° 22 – O2 If /WN5

(Fig. B.7). This star was re-classified30 as O2 If/WN 5 by Crowther & Walborn (2011) using their updated classification scheme, because of the Hβ P-Cygni profile. Using lines from N iii/N iv/N v, we inferred Teff = 46 kK which is hotter than the lower limit (from very weak He iλ4471) quoted by Mas05. Our fit seems to slightly overpredict the emission in N ivλ4058 and to underpredict the N v doublet. An extreme nitrogen abundance, [N] = 8.78, was required, the largest one found in our sample. Such an abundance would be certainly too large when comparing even with strongly nitrogen-enhanced O-stars, and also with predictions from evolutionary calculations tailored for the LMC (Brott et al. 2011b and paper II), thus supporting a rather evolved nature of this object and its “slash-star” designation. This star was also analyzed by Doran & Crowther (2011) using N iv/N v lines (without discussion of He i and N iii), only providing a Teff = 49.3 kK for this object. Such hotter temperature would improve our fits for N ivλ4058 and the N v doublet, but is inconsistent with the observed strength of N iii.

LH 101:W3-19 – O2 If

(Fig. B.8). For this supergiant, a consistent He/N analysis allowed us to derive Teff = 44 kK, hotter than the lower limit (marginal HeIλ4471) assigned by Mas05. Using N iii/N iv/N v in parallel, we achieved an almost excellent fit for the nitrogen lines at [N] = 8.18.

Sk–65 ° 47 – O4 If

(Fig. B.9). The parameter set derived for this star using H/He/N lines is quite similar to the results from Mas05, with somewhat larger YHe = 0.12, A potential discrepancy provides N ivλ4058, where we might slightly overpredict the observed emission.

B.2. SMC stars

AV 435 – O3 V((f ))

(Fig. B.10). The only discrepancies found during our analysis correspond to an overprediction of He iiλλ6406, 6527, 6683. Both the He i/He ii and the N iii/N iv ionization equilibrium suggest a hotter temperature than quoted by Mas05, Teff = 46 kK. This temperature seems to be somewhat cool for its spectral type O3 V assigned by Mas05 because of N ivλ4058  ≳  N iiiλ4640, but quite consistent with our predictions for the derived wind-strength and nitrogen content, [N] = 7.58 (cf. Figs. 7 and 8).

AV 177 – O4 V((f))

(Fig. B.11). The H/He analysis of this star produced similar parameters as found by Mas05. Owing to a high rotation, vsini = 220 km   s-1, nitrogen lines are barely visible in the spectrum. Weak traces of emission at N iiiλλ4634 − 4640 − 4642 and N iiiλλ4510 − 4514 − 4518 together with weak absorption at N ivλ6380 are fitted consistently at [N] = 7.78.

NGC 346-355 – ON2 III(f )

(Fig. B.12). This star was considered as a standard for the O2 III(f) category by Walborn et al. (2002), and later on updated to ON2 III(f) by Walborn et al. (2004). As for N11-031 (same type!) analyzed in paper II, we found problems to fit all N iii/N iv/N v lines in parallel, but to a lesser extent. The basic difference is related to He iλ4471, which is not as clearly visible as for N11-031. During our analysis, we considered two possible parameter sets: a cooler solution with Teff = 51 kK (red) and a hotter one with Teff = 55 kK (black),

using either the N iii/N iv or the N iv/N v ionization equilibrium. By inspection of He iλ4471, we note that both temperatures might be consistent with the very weak observed feature. For a similar nitrogen abundance, [N] = 7.98, we were able to fit either N iiiλλ4634 − 4640 − 4642, N iiiλλ4510 − 4514 − 4518  and N ivλ6380 for the cooler solution, or N ivλ6380 and N vλλ4603-4619 for the hotter one.

Mas09, restricted to He iλ4471 as a primary temperature indicator, derived Teff = 49.5 kK and log g = 3.9, which would agree with our cool solution, but is insufficient for the N iv/N v lines. The hotter solution is in better agreement with results from Bouret et al. (2003) and Walborn et al. (2004), who found Teff = 52.5 kK and log g = 4.0 fitting the N iv/N v lines by means of cmfgen. In particular, we achieved a similar fit quality as Bouret et al. (2003), for a similar nitrogen content. Bouret et al. stated that at Teff  ~  55 kK (identical with our hotter solution) their fit for He iiλ4686 would improve. Such an increase in temperature would also improve their fit of N v, which we are able to fit accurately. The same stellar parameters as determined by Bouret et al. (2003) and Walborn et al. (2004) were derived by Heap et al. (2006) using tlusty, mostly based on lines from highly ionized species, in particular N v and N iv. Unfortunately, they did not comment on He i and N iii, but reassuringly they derived a nitrogen abundance very similar to ours, [N] = 7.92.

thumbnail Fig. B.1

R136-040 - O2-3.5 V. Observed (green) and best fitting optical H/He and N spectrum. For the R136 O-stars, observed spectra for Hα, Hγ, He iλλ4387, 4471, 6678, He iiλ4541, 6406, 6527, 6683, N iiiλλ4510 − 4514 − 4518, and N ivλ6380 taken from the STIS/CCD dataset. Remaining, lower quality spectra collected by FOS. Hβ was not observed for this star.

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thumbnail Fig. B.2

LH 81:W28-23 - O3.5 V((f+)).

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thumbnail Fig. B.3

LH 101:W3-24 - O3.5 V((f+)).

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thumbnail Fig. B.4

LH 81:W28-5 - O4 V((f+)).

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thumbnail Fig. B.5

R136-018 - O3 III(f). Observations as for R136-040 (Fig. B.1).

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thumbnail Fig. B.6

LH 90:ST 2-22 - O3.5 III(f+).

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thumbnail Fig. B.7

Sk–67° 22 - O2 If/WN5.

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thumbnail Fig. B.8

LH 101:W3-19 - O2 If.

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thumbnail Fig. B.9

Sk–65° 47 - O4 If.

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thumbnail Fig. B.10

AV 435 - O3 V((f)).

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thumbnail Fig. B.11

AV 177 - O4 V((f)).

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thumbnail Fig. B.12

NGC 346-355 - ON2 III(f). Black: hotter solution (Teff = 55 kK), supported by N iv/N v lines. Blue: cooler solution (Teff = 51 kK), mostly supported by N iii (together with N ivλ6380). Red: “average” solution (Teff = 53 kK) used in Sect. 7.

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© ESO, 2012

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