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.

arXiv comment10 pages, 1 figure, ApJL in press

References in corpus (37)

• Intrinsic Colors, Temperatures, and Bolometric Corrections of Pre-Main Sequence Stars
• A Grid of NLTE Line-Blanketed Model Atmospheres of O-type Stars
• A Grid of NLTE Line-Blanketed Model Atmospheres of Early B-type Stars
• Be/X-ray binaries
• Classical Be Stars: Rapidly Rotating B Stars with Viscous Keplerian Decretion Disks
• Nominal values for selected solar and planetary quantities: IAU 2015 Resolution B3
• On the Spectral Evolution of Hot White Dwarf Stars. I. A Detailed Model-Atmosphere Analysis of Hot White Dwarfs from SDSS DR12
• Spectral models for binary products: Unifying Subdwarfs and Wolf-Rayet stars as a sequence of stripped-envelope stars
• Detached white dwarf main-sequence star binaries
• The expansion of stripped-envelope stars: consequences for supernovae and gravitational-wave progenitors
• The evolution of naked helium stars with a neutron-star companion in close binary systems
• Detection of additional Be+sdO systems from IUE spectroscopy
• The Detection and Characterization of Be+sdO Binaries from HST/STIS FUV Spectroscopy
• The X-ray emission of the gamma Cassiopeiae stars
• A Study of Pi Aquarii During a Quasi-normal Star Phase: Refined Fundamental Parameters and Evidence for Binarity
• Clues about the scarcity of stripped-envelope stars from the evolutionary state of the sdO+Be binary system phi Persei
• 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
• Three discoveries of gamma Cas analogs from dedicated XMM-Newton observations of Be stars
• Suzaku and NuSTAR X-Ray Spectroscopy of gamma Cas and HD110432
• 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)
• Doppler tomography of the circumstellar disk of π Aquarii
• The X-ray emission of Be+stripped star binaries
• Spectral Energy Distributions of Be and Other Massive Stars
• Is there a propeller neutron star in $\gamma$ Cas?
• Surprises in the simultaneous X-ray and optical monitoring of pi Aqr
• FUV Spectroscopy of the Dwarf Novae SS Cygni and WX Hydri in Quiescence
• Detecting the Companions and Ellipsoidal Variations of RS CVn Primaries: II. omicron Draconis, a Candidate for Recent Low-Mass Companion Ingestion
• MICADO: The Multi-Adaptive Optics Camera for Deep Observations
• Spectroscopic Detection of the Pre-White Dwarf Companion of Regulus
• X-ray and optical spectroscopic study of a gamma Cassiopeiae analog source pi Aquarii
• Progenitor neutron stars of the lightest and heaviest millisecond pulsars

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• Fluorescent Fe K line emission of gamma Cas stars II. Predictions for magnetic interactions and white-dwarf accretion scenarios