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Repolust, Tamara (2005): Quantitative spectroscopy of OB-stars in the optical and the infrared. Dissertation, LMU München: Faculty of Physics
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Abstract

Massive OB stars are the most luminous stellar objects (10e5 to a few 10e6 L⊙). Although being rare by number they play a dominant role in the chemical and dynamical evolution of galaxies through their input of energy, momentum, and nuclear processed material into the interstellar medium by means of stellar winds, eruptions, and (final) explosions. The luminosity of hot massive stars is the key ingredient to the driving of a dense (10e−6 - 10e−5 M⊙/yr) and fast (up to 3,000 km/s) outflow lasting a lifetime. This mass loss imprints unambiguous signatureson the spectral energy distribution and spectral lines received from these objects. The goal of this thesis was to investigate and to apply recent, improved methods for spectral diagnostics in the optical and the infrared by means of unified model atmospheres, comprising the entire sub- and supersonic structure from the pseudo-hydrostatic photosphere to the stellar wind. In particular, we used the nlte-model atmospheres code fastwind, which is highly computational efficient and updated to comprise an adequate though approximate treatment of metal line opacity effects, i.e., metal line-blocking/-blanketing. First we have tested this code by a comparison with alternative codes (e.g., cmfgen by Hillier & Miller 1998 and WM-Basic by Pauldrach et al. 2001), particularly in terms of temperature stratification, fluxes, and number of ionizing photons. In almost all cases we obtained very similar results. Also for the H/He lines which could be only compared to cmfgen, the coincidence between the codes is remarkable, except for a subtle discrepancy concerning the He i singlets, where, in a restricted temperature range, cmfgen predicts weaker singlet lines. Having tested the improved model atmospheres code we began our study with a re-analysis of the Galactic O-star sample presented by Puls et al. 1996 (at that time using pure H/He models) to investigate the influence of line-blocking/-blanketing. This re-analysis (by means of profile fitting of photospheric and wind lines from H and He) resulted in a significant re-definition of the effective temperature scale due to this line-blanketing effect. We obtain lower effective temperatures (up to 8,000 K, depending on spectral type and luminosity class) in combination with a reduction in either gravity or helium abundance, thus, making it possible to assign a new Teff - log g and Teff - spectral type calibration as a function of luminosity class. Further, by calculating new spectroscopic masses and comparing them with previous results we find a significant reduction in the so-called mass discrepancy (Herrero et al. 1992), where the latter describes the unfortunate situation that spectroscopically derived masses are lower than those resulting from stellar evolution calculations. For stars below 50 M⊙ a systematic trend is retained such that the spectroscopically derived masses are smaller by approx. 10 M⊙ compared to the evolutionary ones. Moreover, the wind momentum luminosity relation (WLR) changes because of lower luminosities and almost unmodified wind-momentum rates. Still present, however, is a separation of the WLR as a function of luminosity class, in contrast to theoretical simulations which do not predict such a dependence. From simple arguments and using stellar samples of different sizes, we find strong indications that for most supergiants the mass-loss rate is over-estimated by a factor of 2 to 3, whereas the mass-loss-estimates for luminosity class III and V objects are consistent with our own theoretical expectations and those by others. The over-estimate is interpreted as an effect of wind-clumping, and our argumentation is based on the assumption that the material in the lower wind region is un-clumped, in accordance with theoretical predictions. As a final step we have analyzed a large sample of OB stars by means of H and K band spectroscopy in the infrared (IR) regime, with the primary goal to investigate to what extent a lone near IR-spectroscopy is able to recover stellar and wind parameters derived in the optical. Due to the substantial progress in ground-based IR instrumentation in the past decade and the extension of model atmosphere codes to the infrared wavelength regime, IR spectroscopy has become a powerful diagnostics for the investigation of young and massive stars lying deeply embedded in the dust-enshrouded environment of molecular clouds or the Galactic centre, allowing us to take a first step in the direction of a pure IR analysis. For the stars analyzed we obtain well-agreeing results between the optical and the infrared, except for the line cores of Br gamma in early O stars with significant mass loss, which, again, might indicate the presence of clumping effects. Having derived the stellar and wind parameters from the IR, we are now able to constrain the observational requirements to perform a pure IR-analysis. Most important is a very high S/N ratio, as the lines to be investigated are extremely shallow, and a very good resolution, in addition to an adequately large set of strategic lines. Given this prerequisite, spectral analyses based on pure IR data could, indeed, be successfully used as an alternative or support to traditional methods, and will allow us to proceed towards our ultimate goal of analyzing very young and highly obscured objects just emanating from their birth places.