A non-LTE analysis of a sample of O stars selected from galactic OB associations

Abstract

The tables of non-LTE line profiles and equivalent widths published by Mihalas and his collaborators [33], [7], [35] have been revised and extended to four different values of the abundance ratio He/H. Bolometric corrections have been calculated for V magnitudes. The theoretical line profiles have been fit to echelle spectrograms of 22 galactic O stars by X2 minimization. It is found that the stars with the lowest surface gravities are fitted best by theoretical spectra with unexpectedly high helium abundances (He/H~0.50), while the stars with higher surface gravities are fitted best by theoretical spectra with He/H~0.10, the accepted cosmic ratio. This suggests a systematic failure of conventional non-LTE, plane-parallel models for the more luminous O stars, probably as a result of the neglect of geometrical dilution. The formula, log He/H = 1.1234-0.4791 log g, gives a good fit to the relation between the apparent helium abundance and log g. Using this relationship, the apparent abundances have been reduced to what are probably true abundances relative to the normal cosmic abundance. It is found that there is no significant difference in the average helium abundances of the associations observed. However, the stars HD 12993, HD 242908, and HD 193595 may be blue stragglers with moderately enhanced helium abundance (He/H~0.19). Relative carbon abundances have been determined empirically by comparison of the CIV 5812A and HeII 4542A equivalent widths. It is found that the association Cyg 0B2 is overabundant in carbon by ~50%. Likewise, the blue straggler HD 236894 is underabundant in carbon by a factor of two. The estimated effective temperatures of the sample are compared to the previously accepted calibration of MK spectral types to the effective temperature. Estimates of the radii and masses of the stars in the sample have been calculated from their physical parameters and their absolute visual magnitudes.