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Rivero González, Jorge Gustavo (2012): Nitrogen line spectroscopy in O-stars. Dissertation, LMU München: Faculty of Physics
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Abstract

This work aims at enabling quantitative optical nitrogen line spectroscopy in O-stars, in order to improve our knowledge about these objects, particularly their earliest spectral subtypes. To this end, nitrogen has proven to be a key element, both in terms of its potential to infer effective temperatures, and for being the best tracer for testing the effects of rotational mixing in massive stellar models, allowing us to further constrain the evolution of massive stars. To accomplish this study, we used the NLTE (non local thermodynamic equilibrium) atmosphere/spectrum synthesis code FASTWIND, and developed a new nitrogen model atom, partly based on older work, comprising the ionization stages NII to NV. Moreover, we incorporated the dielectronic recombination mechanism into FASTWIND, which was previously unable to deal with this process. We performed an extensive investigation on the line-formation process of strategic nitrogen emission lines such as NIII4640 and NIV4058, accounting for a complete treatment of line blocking/blanketing effects and the presence of a wind, as it is possible when using a state-of-the-art atmospheric code. Contrasted to the results from the seminal work on the NIII triplet formation performed by Mihalas & Hummer (1973), based on much simpler model atmospheres, our study implies that dielectronic recombination plays only a secondary role under Galactic conditions. Rather, the emission is controlled by the stellar wind (Swings mechanism), as long as the wind is powerful enough to enable a significantly accelerating velocity field already in the photospheric formation region. For later spectral O-subtypes, the strength of the emission might be also affected by an OIII resonance line overlapping with the NIII resonance line in the EUV. Concerning the emission at NIV4058, we suggest a rather similar mechanism. Also in this case, the dominating process is the strong depopulation of the lower level of the transition, which increases as a function of the wind-strength. Unlike the NIII triplet emission, however, resonance lines do not play a role for typical mass-loss rates and below. Using the updated version of FASTWIND and our new model atom, we derived nitrogen abundances for a substantial O-star sample in the Large Magellanic Cloud (LMC). Stellar and wind parameters of our sample stars were determined by line profile fitting of hydrogen, helium and nitrogen lines, exploiting the corresponding ionization equilibria. The bulk of our sample stars turned out to be strongly nitrogen-enriched, and a clear correlation of nitrogen and helium enrichment was found. By comparing the nitrogen abundances as a function of projected rotation velocity with tailored evolutionary calculations, we identified a considerable number of highly enriched, but slowly rotating object, which should not exist according to standard theory of rotational mixing. Our findings support the basic outcome of previous B-star studies within the VLT-FLAMES survey on massive stars which pointed to similar discrepancies. The detection of strong nitrogen enrichment in the bulk of our sample stars indicates that efficient mixing takes place already during the very early phases of stellar evolution of LMC O-stars. In the last part of this thesis, we concentrated on the applicability of the Walborn et al. (2002) classification scheme for very early O-stars, which is primarily based on the relative strengths of the NIV4058 and NIII4640 emission lines. This scheme has already been used in a variety of studies, but is still disputed for various reasons. We provided first theoretical predictions on the NIV4058/NIII4640 emission line ratio in dependence of different parameters, and confronted these predictions with results from an analysis of a sample of early-type LMC/SMC O-stars. Though we found a monotonic relationship between the NIV/NIII emission line ratio and the effective temperature, all other parameters being equal, our predictions indicate additional dependencies on other important stellar parameters, most significantly, the nitrogen abundance. The relation between the observed NIV/NIII emission line ratio and the effective temperature, for a given luminosity class, turned out to be quite monotonic for our sample stars, and to be fairly consistent with our model predictions. The scatter within a spectral subtype is mainly produced by abundance effects. Our findings suggest that the Walborn et al. classification scheme is able to provide a meaningful relation between spectral type and effective temperature, as long as it is possible to discriminate for the luminosity class.