A new calibration of stellar parameters of Galactic O stars

¹ Observatoire de Genève, 51 Chemin des Maillettes, 1290 Sauverny, Switzerland e-mail: martins@mpe.mpg.de
² Laboratoire d'Astrophysique, Observatoire Midi-Pyrénées, 14 Av. E. Belin, 31400 Toulouse, France
³ Department of Physics and Astronomy, University of Pittsburgh, 3941 O'Hara Street, Pittsburgh, PA 15260, USA

Received: 18 November 2004
Accepted: 14 March 2005

Abstract

We present new calibrations of stellar parameters of O stars at solar metallicity taking non-LTE, wind, and line-blanketing effects into account. Gravities and absolute visual magnitudes are derived from results of recent spectroscopic analyses. Two types of effective temperature scales are derived: one from a compilation based on recent spectroscopic studies of a sample of massive stars – the “observational scale” – and the other from direct interpolations on a grid of non-LTE spherically extended line-blanketed models computed with the code CMFGEN (Hillier & Miller 1998) – the “theoretical scale”. These T_eff scales are then further used together with the grid of models to calibrate other parameters (bolometric correction, luminosity, radius, spectroscopic mass and ionising fluxes) as a function of spectral type and luminosity class. Compared to the earlier calibrations of Vacca et al. (1996) the main results are: 
[–] The effective temperature scales of dwarfs, giants and supergiants are cooler by 2000 to 8000 K, the theoretical scale being slightly cooler than the observational one. The reduction is the largest for the earliest spectral types and for supergiants.
[–] Bolometric corrections as a function of T_eff are reduced by 0.1 mag due to line blanketing which redistributes part of the UV flux in the optical range. For a given spectral type the reduction of BC is larger for early types and for supergiants. Typically BCs derived using the theoretical T_eff scale are 0.40 to 0.60 mag lower than that of Vacca et al. (1996), whereas the differences using the observational T_eff scale are somewhat smaller.
[–] Luminosities are reduced by 0.20 to 0.35 dex for dwarfs, by ~0.25 for all giants and by 0.25 to 0.35 dex for supergiants. The reduction is essentially the same for both T_eff scales. It is independent of spectral type for giants and supergiants and is slightly larger for late type than for early type dwarfs.
[–] Lyman continuum fluxes are reduced. Our theoretical values for the hydrogen ionising photon fluxes for dwarfs are 0.20 to 0.80 dex lower than those of Vacca et al. (1996), the difference being larger at late spectral types. For giants the reduction is of 0.25 to 0.55 dex, while for supergiants it is of 0.30 to 0.55 dex. Using the observational T_eff scale leads to smaller reductions at late spectral types.
 
The present results should represent a significant improvement over previous calibrations, given the detailed treatment of non-LTE line-blanketing in the expanding atmospheres of massive stars.