Gamma Cas Stars as Be + White Dwarf Binary Systems

TL;DR

This paper explores the hypothesis that gamma Cas stars are binary systems with a Be star and a faint white dwarf companion, supported by models and interferometric observations that favor white dwarfs over other possible companions.

Contribution

It provides model predictions and observational evidence supporting white dwarfs as companions in gamma Cas systems, clarifying their physical and orbital properties.

Findings

Current observations are consistent with white dwarf companions.

Interferometry shows no evidence of bright companions, supporting faint white dwarf hypothesis.

Hot subdwarf companions are unlikely due to lack of ultraviolet signatures.

Abstract

The origin of the bright and hard X-ray emission flux among the gamma Cas subgroup of B-emission line (Be) stars may be caused by gas accretion onto an orbiting white dwarf (WD) companion. Such Be+WD binaries are the predicted outcome of a second stage of mass transfer from a helium star mass donor to a rapidly rotating mass gainer star. The stripped donor stars become small and hot white dwarfs that are extremely faint compared to their Be star companions. Here we discuss model predictions about the physical and orbital properties of Be+WD binaries, and we show that current observational results on gamma Cas systems are consistent with the expected large binary frequency, companion faintness and small mass, and relatively high mass range of the Be star hosts. We determine that the companions are probably not stripped helium stars (hot subdwarf sdO stars), because these are bright enough to detect in ultraviolet spectroscopy, yet their spectroscopic signatures are not observed in studies of gamma Cas binaries. Interferometry of relatively nearby systems provides the means to detect very faint companions including hot subdwarf and cooler main sequence stars. Preliminary observations of five gamma Cas binaries with the CHARA Array interferometer show no evidence of the companion's flux, leaving white dwarfs as the only viable candidates for the companions.