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Astron. Astrophys. 320, 500-524 (1997)

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4. Model results

In this section we summarise the technique used while we describe the results of our model fits for individual stars in Appendix A. Further details of the model atmosphere used, diagnostic lines employed, and the method adopted to obtain optimal line profile matches are described at length in Paper I.

4.1. Technique

The model calculations are based on the iterative technique of Hillier (1987, 1990) which solves the transfer equation in the co-moving frame subject to statistical and radiative equilibrium, assuming an expanding, spherically-symmetric, homogeneous and static atmosphere. The stellar radius ([FORMULA]) is defined as the inner boundary of the model atmosphere and is located at Rosseland optical depth of [FORMULA] 20 with the stellar temperature ([FORMULA]) defined by the usual Stefan-Boltzmann relation. Similarly, the effective temperature ([FORMULA]) relates to the radius ([FORMULA]) at which the Rosseland optical depth equals 2/3. We adopt the usual [FORMULA] =1 velocity law for all our analyses.

For the present application, the adopted model atom incorporates 10 levels of H I ([FORMULA] 10), 16 of He II ([FORMULA] 16) and 39 levels of He I ([FORMULA] 14), and is almost identical to that used in Paper II. Although metal abundance determinations are not considered here, we also incorporate simplified atoms of carbon (C III-IV) and nitrogen (N II-V) since these have a dominant effect on the cooling of the wind (Hillier 1988). For this study we adopt carbon and nitrogen abundances of N/He=0.0025, C/N=0.1 which represent mean values for the LMC WN9-10 stars from Paper I. In total, 95 individual levels and 521 non-LTE transitions are simultaneously considered.

Following Papers I-III, diagnostic He I ([FORMULA] 5876), He II ([FORMULA] 4686 or [FORMULA] 5412) and H I (H [FORMULA], H [FORMULA]) lines are used to determine the stellar parameters. While the He II diagnostic is usually [FORMULA] 5412, we use [FORMULA] 4686 instead for very low excitation Wolf-Rayet stars (WN9-11) because of the weakness of the former. As discussed in Sect. 3.3, the availability of AAT-UCLES and MSO-coudé observations for S119 and S61 allows a further test of our results for those (typically WN11) stars with possible Balmer line nebular contamination. Radial velocities for individual stars are obtained from line profile fits ([FORMULA] [FORMULA] 20 km s-1).

Uncertainties in the derived physical parameters for WNL stars have previously been described in Papers I-II, in which we have also investigated the effect of line blanketing. In particular, inconsistencies between hydrogen-helium and metal analyses suggest that stellar temperatures and luminosities are underestimated using non-blanketed hydrogen-helium models. In the extreme case of WR25 (HD 93162 WN6ha), use of a nitrogen rather than hydrogen-helium analysis, led to significant increases in stellar luminosity (0.2 dex), bolometric correction (0.6 dex), H0 ionizing photon luminosity (0.5 dex), and stellar temperature (7 000K). It is hoped that future studies incorporating line blanketing will reconcile such differences. Nevertheless, since we currently follow an identical approach to Papers I-II, we are confident that internal comparisons of fundamental parameters for Galactic and LMC stars remain valid. Future photoionization model studies of WR nebulae (e.g. Esteban et al. 1993) would provide a rigorous check on theoretical predictions.

Line profile fits to diagnostic lines are presented in the Appendix (Figs. 7-10) where we also discuss individual objects, arranged in increasing WNL subtype.

The derived stellar parameters for our programme stars are presented in Table 2 and compared with previous Escape Probability Method (L.J. Smith & Willis 1983) and pure helium standard model (Koesterke et al. 1991, Vacca 1992) analyses in Table 3. Overall we find slightly higher stellar luminosities relative to pure helium analyses, although this effect is enhanced due to the somewhat higher absolute visual magnitudes obtained here. As discussed in Paper II for Galactic WNL stars, bolometric corrections incorporating hydrogen and metals are typically [FORMULA] 0.2 mag higher than pure helium models, while stellar and ionizing photon luminosities are [FORMULA] 0.2 dex higher.


Table 2. Derived stellar parameters for the programme LMC WN6-11 stars, including four WN9-10 stars from Paper I (indicated by [FORMULA]). Two entries are given for S119: Model A using our AAT-RGO spectra including Balmer nebular contributions and Model B, based on our AAT-UCLES and MSO-coudé spectra, with Balmer nebular contaminations removed. We include Lyman continuum ionising photons, [FORMULA] and wind performance numbers, [FORMULA]


Table 3. Comparison of stellar parameters derived here (CS) with previous analyses (L.J. Smith & Willis 1983 SW; Koesterke et al. 1991 KHSW; Vacca 1992 WDV)

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998