The Effective Temperatures of Mid‐O Stars

Abstract

We derived photospheric parameters, mass-loss rates, and wind velocitiesof Galactic O6-O7 stars by analyzing high-resolution spectra in thefar-UV and UV ranges with line-blanketed, hydrodynamic, non-LTEspherical models. We combined spectra from the Far UltravioletSpectroscopic Explorer (FUSE) in the range 905-1187 {Å} andInternational Ultraviolet Explorer (IUE) archival spectra (1150-3250{Å}) and used the WM-BASIC code of Pauldrach et al. to compute modelspectra. Lines in the FUSE range include high ionization stages (e.g., OVI) and lower abundance non-CNO elements (e.g., P V). Analyzed inaddition to the N IV, N V, Si IV, and C IV lines in the IUE range, thesefeatures play a crucial role in uniquely constraining the stellarparameters, assessing the presence of shocks in the wind, andquantifying the effects of the resulting soft X-rays on the windionization. The effective temperatures derived from the consistentanalysis of the far-UV and UV spectra are significantly (~6000 K or 15%on average, or between 4000 and 8000 K) lower than most valuespreviously derived for some of our targets and lower than typical valuesassigned to their spectral types from different compilations. Thisresult has great implications for our understanding of the evolution ofmassive stars and the characterization of young stellar populations, aswell as for energy balance calculations of H II regions. Based onobservations with the NASA-CNES-CSA Far Ultraviolet SpectroscopicExplorer, which is operated by The Johns Hopkins University under NASAcontract NAS5-32985, and on IUE observations from the MAST and INESarchives.