gamma Cas stars: Normal Be stars with disks impacted by the wind of a helium-star companion?

Abstract

$\gamma$ Cas stars are a $\sim$1% minority among classical Be stars with hard but only moderately strong continuous thermal X-ray flux and mostly very early-B spectral type. The X-ray flux has been suggested to originate from matter accelerated via magnetic disk-star interaction, by a rapidly rotating neutron star (NS) companion via the propeller effect, or by accretion onto a white dwarf (WD) companion. In view of the growing number of identified $\gamma$ Cas stars and the only imperfect matches between these suggestions and the observations, alternative models should be pursued. Two of the three best-observed $\gamma$ Cas stars, $\gamma$ Cas itself and $\pi$ Aqr, have a low-mass companion with low optical flux; interferometry of BZ Cru is inconclusive. Binary-evolution models are examined for their ability to produce such systems. The OB+He-star stage of post-mass transfer binaries, which is otherwise observationally unaccounted, can potentially reproduce many observed properties of $\gamma$ Cas stars. The interaction of the fast wind of helium stars with the disk and/or with the wind of Be stars may give rise to the production of hard X-rays. While not modelling this process, it is shown that the energy budget is favourable, and that the wind velocities may lead to hard X-rays as observed in $\gamma$ Cas stars. Furthermore, their observed number appears to be consistent with the evolutionary models. Within the Be+He-star binary model, the Be stars in $\gamma$ Cas stars are conventional classical Be stars. They are encompassed by O-star+Wolf-Rayet systems towards higher mass, where no stable Be decretion disks exist, and by Be+sdO systems at lower mass where the sdO winds may be too weak to cause the $\gamma$ Cas phenomenon. In decreasing order of the helium-star mass, the descendants could be Be+black-hole, Be+NS or Be+WD binaries.

arXiv comment16 pages, 7 figures, Astronomy and Astrophysics, in press, Abstract abridged

References in corpus (75)

• The Wide-field Infrared Survey Explorer (WISE): Mission Description and Initial On-orbit Performance
• The SIMBAD astronomical database
• The VizieR database of Astronomical Catalogues
• Pre-Supernova Evolution of Massive Single and Binary Stars
• Rotating Massive Main-Sequence Stars I: Grids of Evolutionary Models and Isochrones
• Be/X-ray binaries
• Classical Be Stars: Rapidly Rotating B Stars with Viscous Keplerian Decretion Disks
• The rotation rates of massive stars: the role of binary interaction through tides, mass transfer and mergers
• Magnetorotational Instability in Protoplanetary Disks. II. Ionization State and Unstable Regions
• The incidence of stellar mergers and mass gainers among massive stars
• A natural explanation for periodic X-ray outbursts in Be/X-ray binaries
• X-Ray Spectroscopy of Stars
• Hydrodynamical Simulations of Corotating Interaction Regions and Discrete Absorption Components in Rotating O-Star Winds
• Formation of contact in massive close binaries
• Massive runaways and walkaway stars
• The Evolution of Massive Helium Stars Including Mass Loss
• Constraining the mass transfer in massive binaries through progenitor evolution models of Wolf-Rayet+O binaries
• A Be-type star with a black hole companion
• Spectral models for binary products: Unifying Subdwarfs and Wolf-Rayet stars as a sequence of stripped-envelope stars
• Efficiency of mass transfer in massive close binaries, Tests from double-lined eclipsing binaries in the SMC
• First Results from the CHARA Array. I. An Interferometric and Spectroscopic Study of the Fast Rotator alpha Leonis (Regulus)
• Magnetic spots on hot massive stars
• High-Mass X-ray binaries in the Small Magellanic Cloud
• Effects of metallicity, star-formation conditions, and evolution in B and Be stars. II: Small Magellanic Cloud, field of NGC 330
• Mass loss in main-sequence B stars
• Detection of additional Be+sdO systems from IUE spectroscopy
• Winds from stripped low-mass Helium stars and Wolf-Rayet stars
• CHARA Array K'-band Measurements of the Angular Dimensions of Be Star Disks
• Populations of rotating stars II. Rapid rotators and their link to Be-type stars
• Spectroscopic and physical parameters of Galactic O-type stars. III. Mass discrepancy and rotational mixing
• On the physical nature of accretion disc viscosity
• The life cycles of Be viscous decretion discs: fundamental disc parameters of 54 SMC Be stars
• The X-ray emission of the gamma Cassiopeiae stars
• FUV Detection of the Suspected Subdwarf Companion to the Be Star 59 Cygni
• Spectral and spatial imaging of the Be+sdO binary phi Persei
• A Study of Pi Aquarii During a Quasi-normal Star Phase: Refined Fundamental Parameters and Evidence for Binarity
• First spectro-interferometric survey of Be stars I. Observations and constraints on the disks geometry and kinematics
• Disk Loss and Disk Renewal Phases in Classical Be Stars I: Analysis of Long-Term Spectropolarimetric Data
• Detection of the Ultraviolet Spectrum of the Hot Subdwarf Companion of 60 Cygni (B1 Ve) from a Survey of IUE Spectra of Be Stars
• On the kinematics of a runaway Be star population
• X-ray and Optical Variations in the Classical Be Star gamma Cas
• Constraining Disk Parameters of Be Stars using Narrowband H-alpha Interferometry with the NPOI
• The Hot Companion and Circumbinary Disk of the Be Star HR 2142
• Hot stars observed by XMM-Newton II. A survey of Oe and Be stars
• A Spectroscopic Orbit for Regulus
• The life cycles of Be viscous decretion discs: The case of ω CMa
• The Past and Future History of Regulus
• Non-thermal processes in bowshocks of runaway stars. Application to Zeta Oph
• The relationship between gamma Cassiopeiae's X-ray emission and its circumstellar environment
• The Tarantula Massive Binary Monitoring. IV. Double-lined photometric binaries
• Be discs in coplanar circular binaries: Phase-locked variations of emission lines
• A propelling neutron star in the enigmatic Be-star $\gamma$~Cassiopeia
• Outbursts and Disk Variability in Be Stars
• Short-term variability and mass loss in Be stars II. Physical taxonomy of photometric variability observed by the Kepler spacecraft
• Colliding Stellar Winds Structure and X-ray Emission
• Suzaku and NuSTAR X-Ray Spectroscopy of gamma Cas and HD110432
• gamma Cassiopeiae: an X-ray Be star with personality
• Discovery of Rapidly Moving Partial X-ray Absorbers within gamma Cassiopeiae
• The Remarkable Be Star HD110432
• Doppler tomography of the circumstellar disk of π Aquarii
• Disruption of circumstellar discs by large-scale stellar magnetic fields
• Rotational and Cyclical Variability in gamma Cassiopeiae. II. Fifteen Seasons
• An investigation of the photometric variability of confirmed and candidate Galactic Be stars using ASAS-3 data
• Is there a propeller neutron star in $\gamma$ Cas?
• IR nebulae around bright massive stars as indicators for binary interactions
• The spin rates and spin evolution of the O components in WR+O binaries
• pi Aqr is another gamma-Cas object
• High-Energy Properties of the Enigmatic Be Star gamma Cassiopeiae
• Characterization of the X-ray light curve of the gamma Cas-like B1e star HD110432
• Evidence of an asymmetrical Keplerian disk in the Brγ and He I emission lines around the Be star HD 110432
• Ultraviolet Activity as indicator of Small-scale Magnetic Fields in gamma Cassiopeiae
• The VLT-FLAMES Tarantula Survey. XXIII: two massive double-lined binaries in 30~Doradus
• Supersoft X-ray sources identified with Be binaries in the Magellanic Clouds
• Variations of the HeII 1640A Line in B0e--B2.5e Stars
• Search for systemic mass loss in Algols with bow shocks

Cited by in corpus (31)

• Polluting the pair-instability mass gap for binary black holes through super-Eddington accretion in isolated binaries
• A naked-eye triple system with a nonaccreting black hole in the inner binary
• An X-ray quiet black hole born with a negligible kick in a massive binary within the Large Magellanic Cloud
• The Detection and Characterization of Be+sdO Binaries from HST/STIS FUV Spectroscopy
• Investigating the lack of main-sequence companions to massive Be stars
• The Tarantula Massive Binary Monitoring VI: Characterisation of hidden companions in 51 single-lined O-type binaries, a flat mass-ratio distribution, and black-hole binary candidates
• The young massive SMC cluster NGC 330 seen by MUSE II. Multiplicity properties of the massive-star population
• Velocity monitoring of gamma-Cas stars reveals their binarity status
• Population Synthesis of Galactic Be-star Binaries with A Helium-star Companion
• Binarity at LOw Metallicity (BLOeM): a spectroscopic VLT monitoring survey of massive stars in the SMC
• Interferometric detections of sdO companions orbiting three classical Be stars
• HR 6819 is a binary system with no black hole -- revisiting the source with infrared interferometry and optical integral field spectroscopy
• The X-ray emission of Be+stripped star binaries
• The B-type Binaries Characterisation Programme II. VFTS 291: A stripped star from a recent mass transfer phase
• To Be or not to Be: the role of rotation in modeling Galactic Be X-ray Binaries
• Short-term variability and mass loss in Be stars V. Space photometry and ground-based spectroscopy of $\gamma$ Cas
• Ultraviolet Spectropolarimetry: on the origin of rapidly rotating B stars
• The X-ray emission of gamma Cassiopeiae during the 2020-2021 disc eruption
• Short-term variability and mass loss in Be stars VI. Frequency groups in $\gamma$ Cas detected by TESS
• The spins of stripped B stars support magnetic internal angular momentum transport
• Populations of evolved massive binary stars in the Small Magellanic Cloud I: Predictions from detailed evolution models
• X-ray response to disk evolution in two gamma-Cas stars
• Fluorescent Fe K line emission of gamma Cas stars I. Do gamma Cas stars host propelling neutron stars?
• An interferometric study of B star multiplicity
• X-ray emission of massive stars and their winds
• A Time-Dependent Spectral Analysis of $\gamma$ Cassiopeiae
• Automated photometry of $\gamma$ Cassiopeiae:the last roundup
• V1294 Aql = HD 184279: A bad boy among Be stars or an important clue to the Be phenomenon?
• Fluorescent Fe K line emission of gamma Cas stars II. Predictions for magnetic interactions and white-dwarf accretion scenarios
• Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656
• The VLT-FLAMES survey of massive stars. NGC2004#115: A triple system hosting a possible short period B+BH binary