The “Mass Discrepancy” for Massive Stars: Tests of Models Using Spectroscopic Binaries

Abstract

Stellar evolutionary models are often used to infer a star's mass via its luminosity, but empirical checks on the accuracy of the theoretical mass-luminosity relation for very massive stars have been lacking. This is of particular concern given that modern atmosphere models yield systematically smaller masses for massive stars than do evolutionary models, with the discrepancy being a factor of 2 for Of stars. We attempt to resolve this mass discrepancy by obtaining new, high-resolution optical data on seven early-type spectroscopic binaries: V453 Cyg, HD 191201, V382 Cyg, Y Cyg, HD 206267, DH Cep, and AH Cep. Our study produces improved spectral subtypes for the components of these systems, which are crucial for evaluating their luminosities and locations in the H-R diagram. Our radial velocity study utilizes a measuring method that explicitly accounts for the effects of pair blending. We combine our new orbit solutions with existing data on inclinations and distances when available to compare the orbital masses with evolutionary models, and we find good agreement in all cases where the stars are noninteracting. (The components of V382 Cyg and DH Cep fill their Roche lobes, and in both cases we find masses substantially lower than the masses inferred from evolutionary tracks, suggesting that significant material has been lost rather than transferred. We confirm that this same trend exists for other systems drawn from the literature.) Our own data extends to only 15 M☉, although photometric inclination determinations for HD 191201 and HD 206267 should prove possible and will provide examples of higher mass systems. We briefly discuss suitable systems from the literature and conclude that orbit solutions provide good agreement with the evolutionary models to 25 M☉. Beyond this, most known binaries either fill their Roche lobes or have other complications. We also discuss five systems for which our improved data and analysis failed to yield acceptable orbit solutions: EO Aur, IU Aur, V640 Mon (Plaskett's star), LY Aur, and 29 UW CMa all remained intractable, despite improved data.