Gamma Cas Stars as Be + White Dwarf Binary Systems

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.

10 pages, 1 figure, ApJL in press

References in corpus (15)

• A Grid of NLTE Line-Blanketed Model Atmospheres of Early B-type Stars
• On the Spectral Evolution of Hot White Dwarf Stars. I. A Detailed Model-Atmosphere Analysis of Hot White Dwarfs from SDSS DR12
• Detection of the Ultraviolet Spectrum of the Hot Subdwarf Companion of 60 Cygni (B1 Ve) from a Survey of IUE Spectra of Be Stars
• Velocity monitoring of gamma-Cas stars reveals their binarity status
• A propelling neutron star in the enigmatic Be-star $\gamma$~Cassiopeia
• The Dwarf Novae During Quiescence
• Population Synthesis of Galactic Be-star Binaries with A Helium-star Companion
• Discovery of Rapidly Moving Partial X-ray Absorbers within gamma Cassiopeiae
• Interferometric detections of sdO companions orbiting three classical Be stars
• Dynamical masses of the primary Be star and the secondary sdB star in the single-lined binary kappa Dra (B6 IIIe)
• The X-ray emission of Be+stripped star binaries
• Surprises in the simultaneous X-ray and optical monitoring of pi Aqr
• Is there a propeller neutron star in $\gamma$ Cas?
• MICADO: The Multi-Adaptive Optics Camera for Deep Observations
• X-ray and optical spectroscopic study of a gamma Cassiopeiae analog source pi Aquarii

Cited by in corpus (7)

• Fluorescent Fe K line emission of gamma Cas stars I. Do gamma Cas stars host propelling neutron stars?
• Newborn Be star systems observed shortly after mass transfer
• A Be Star + He Star Binary as an Indicator of a Binary Mass Transfer Phase
• A Time-Dependent Spectral Analysis of $\gamma$ Cassiopeiae
• Broad Observational Perspectives Achieved by the Accreting White Dwarf Sciences in the XMM-Newton and Chandra Eras
• Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656
• Fluorescent Fe K line emission of gamma Cas stars II. Predictions for magnetic interactions and white-dwarf accretion scenarios