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تسجيل مستخدم جديدNitrogen line spectroscopy in O-stars -- III. The earliest O-stars
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This is the third paper in a series aiming at the analysis of nitrogen abundances in O-type stars, to enable further constraints on the early evolution of massive stars. We provide first theoretical predictions for the NIV4058/NIII4640 emission line ratio in dependence of various parameters, and confront them with results from the analysis of a sample of early LMC/SMC O-stars. Stellar and wind parameters are determined by line profile fitting of H/He/N lines, exploiting the helium and nitrogen ionization balance. Corresponding synthetic spectra are calculated using the NLTE atmospheric code FASTWIND. Though there is a monotonic relationship between the emission line ratio and Teff, all other parameters being equal, theoretical predictions indicate additional dependencies, most notably, on the nitrogen abundance. These basic predictions are confirmed by results from atmospheric code CMFGEN. The effective temperatures for the earliest O-stars, inferred from the nitrogen ionization balance, are partly considerably hotter than indicated by previous studies. Consistent with earlier results, effective temperatures increase from supergiants to dwarfs for all spectral types in the LMC. The relation between observed NIV4058/NIII4640 emission line ratio and Teff, for a given luminosity class, turned out to be quite monotonic for our sample stars, and fairly consistent with our model predictions. The scatter within a spectral sub-type is mainly produced by abundance effects. Our findings suggest that the Walborn et al. (2002) classification scheme is able to provide a meaningful relation between spectral type and Teff, provided that it is possible to discriminate for the luminosity class. This might be difficult to achieve in low-Z environments such as the SMC, owing to rather low wind-strengths. According to our predictions, the major bias of the classification scheme is due to nitrogen content.
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This is the first paper in a series dealing with optical Nitrogen spectroscopy of O-type stars, aiming at the analysis of Nitrogen abundances. We implemented a new Nitrogen model atom into the NLTE atmosphere/spectrum synthesis code FASTWIND, and c
ompare the resulting optical NIII lines at 4634/40/42 A with other predictions, mostly from Mihalas & Hummer (1973, ApJ 179, 827,`MH), and from the alternative code CMFGEN. Using similar model atmospheres as MH (not blanketed and wind-free), we are able to reproduce their results, in particular the triplet emission lines. According to MH, these should be strongly related to dielectronic recombination (DR) and the drain by certain two-electron transitions. However, using realistic, fully line-blanketed atmospheres at solar abundances, the key role of DR controlling these emission features is superseded -- for O-star conditions -- by the strength of the stellar wind and metallicity. In the case of wind-free models, the resulting lower ionizing EUV-fluxes severely suppress the emission. As the mass-loss rate is increased, pumping through the NIII resonance line(s) in the presence of a near-photospheric velocity field results in a net optical triplet line emission. A comparison with results from CMFGEN is mostly satisfactory, except for the range 30 kK < Teff < 35 kK, where CMFGEN triggers the triplet emission at lower Teff than FASTWIND. This effect could be traced down to line overlap effects between the NIII and OIII resonance lines that so far cannot be simulated by FASTWIND. Since the efficiency of DR and `two electron drain strongly depends on the degree of line-blanketing/-blocking, we predict the emission to become stronger in a metal-poor environment, though lower wind-strengths and Nitrogen abundances might counteract this effect. Weak winded stars should display less triplet emission than stars with `normal winds.
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