The Lyman-Continuum Fluxes and Stellar Parameters of O and Early B-Type Stars

Abstract

Using the results of the most recent stellar atmosphere models applied to a sample of hot stars, we construct calibrations of effective temperature (T<SUB>eff</SUB>), and gravity (log g) with a spectral type and luminosity class for Galactic O-type and early B-type stars. From the model results we also derive an empirical relation between the bolometric correction and T<SUB>eff</SUB> and log g. Using a sample of stars with known distances located in OB associations in the Galaxy and the Large Magellanic Cloud, we derive a new calibration of M<SUB>υ</SUB> with spectral class. With these new calibrations and the stellar atmosphere models of Kurucz, we calculate the physical parameters and ionizing photon luminosities in the H<SUB>0</SUB> and He<SUB>0</SUB> continua for O and early B-type stars. We find substantial differences between our values of the Lyman- continuum luminosity and those reported in the literature. We also discuss the systematic discrepancy between O-type stellar masses derived from spectroscopic models and those derived from evolutionary tracks. Most likely, the cause of this "mass discrepancy" lies primarily in the atmospheric models, which are plane parallel and hydrostatic and therefore do not account for an extended atmosphere and the velocity fields in a stellar wind. Finally, we present a new computation of the Lyman-continuum luminosity from 429 known O stars located within 2.5 kpc of the Sun. We find the total ionizing luminosity from this population (Q<SUB>0</SUB><SUP>Tot</SUP> = 7.0 × 10<SUP>51</SUP> photons s<SUP>-1</SUP>) to be 47% larger than that determined using the Lyman continuum values tabulated by Panagia.