Classical Be Stars: Rapidly Rotating B Stars with Viscous Keplerian Decretion Disks

Abstract

In the past decade, a consensus has emerged regarding the nature of classical Be stars: They are very rapidly rotating main sequence B stars, which, through a still unknown, but increasingly constrained process, form an outwardly diffusing gaseous, dust-free Keplerian disk. In this work, first the definition of Be stars is contrasted to similar classes, and common observables obtained for Be stars are introduced and the respective formation mechanisms explained. We then review the current state of knowledge concerning the central stars as non-radially pulsating objects and non-magnetic stars, as far as it concerns large scale, i.e., mostly dipolar, global fields. Localized, weak magnetic fields remain possible, but are as of yet unproven. The Be phenomenon, linked with one or more mass ejection processes, acts on top of a rotation rate of about 75% of critical or above. The properties of the process can be well constrained, leaving only few options, most importantly, but not exclusively, non-radial pulsation and small scale magnetic fields. Of these, it is well possible that all are realized: In different stars, different processes may be acting. Once the material has been lifted into Keplerian orbit, memory of the details of the ejection process is lost, and the material is governed by viscosity. The disks are fairly well understood in the theoretical framework of the viscous decretion disk model. This is not only true for the disk structure, but as well for its variability, both cyclic and secular. Be binaries are reviewed under the aspect of the various types of interactions a companion can have with the circumstellar disk. Finally, extragalactic Be stars, at lower metallicities, seem more common and more rapidly rotating.

Review accepted for publication in Astronomy and Astrophysics Review, 87 pages with 17 figures

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• Short-term Hα line variations in Classical Be stars: 59 Cyg and OT Gem
• Don't torque like that. Measuring compact object magnetic fields with analytic torque models
• X-ray and optical monitoring of the December 2017 outburst of the Be/X-ray binary AX J0049.4-7323
• A search for Be stars in multiple systems within the solar neighborhood
• 3D MHD Simulation of a Pulsationally-Driven MRI Decretion Disc
• Be Stars in the Open Cluster NGC 6830
• HD 63021: Chromospheric Activity and Mass Transfer in a Close Binary
• Kinematic distinction of the two subpopulations of X-ray pulsars
• Strong Variability in AzV 493, an Extreme Oe-Type Star in the SMC
• The H$\alpha$ line emission of the Be star $\beta$ Psc: the last 40 years
• Recurrent low-level luminosity behaviour after a giant outburst in the Be/X-ray transient 4U 0115+63
• Classical and general relativistic post-Keplerian effects in binary pulsars hosting fast rotating main sequence stars
• Characteristics and evolution of Be-type high mass X-ray binaries as potential Ultraluminous X-ray Sources
• Spectroscopic variability of two Oe stars
• The RCB star V854 Cen is surrounded by a hot dusty shell
• Thermal-timescale accretion does not always yield critical rotation in mass gainers
• Long-term, orbital, and rapid variations of the Be star V923 Aql = HD 183656
• Searching for shell stars in LAMOST DR4 by probing the Fe 42 multiplet lines
• Irregular changes in Hα emission line of V423 Aur observed by LAMOST Medium-Resolution Spectrographs
• Near-infrared Brightening around the Periastron Passages of the Gamma-ray Binary PSR B1259$-$63 /LS 2883
• Red Be stars in the Magellanic Clouds?
• A peculiar interacting Be star binary in the Small Magellanic Cloud
• Study of a Sample of Faint Be Stars in the Exofield of CoRoT. Part III. Global Spectroscopic Characterization and Astrophysical Parameters of the Central Stars
• The SMC B-type supergiant AzV322: a g-mode pulsator with a circumstellar disc
• Exploring membership and variability in NGC 7419: An open cluster rich in super giants and Be type stars
• Unveiling the origin of the optical/UV emission from the Galactic ULX Swift J0243.6+6124 during its 2017-2018 giant outburst
• Photometric and spectroscopic variability of the B5IIIe star HD 171219
• A TESS search for donor-star pulsations in High-Mass X-ray Binaries
• Simultaneous J-, H-, K- and L-band spectroscopic observations of galactic Be stars. I. IR atlas
• AstroSat/UVIT Study of NGC 663: First detection of Be+sdOB systems in a young star cluster
• Angular Momentum Loss Rates in Be Stars Determined by the Viscous Decretion Disc Model
• Proper motion separation of Be star candidates in the Magellanic Clouds and the Milky Way
• On-sky Demonstration of Subdiffraction-limited Astronomical Measurement Using a Photonic Lantern
• Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656
• Spectroscopic study of Be-shell stars: 4 Her and 88 Her
• An eccentric wave in the circumstellar disc of the Be/X-ray binary X Persei
• Fluorescent Fe K line emission of gamma Cas stars II. Predictions for magnetic interactions and white-dwarf accretion scenarios
• Partial, zombie, and full tidal disruption of stars by supermassive black holes
• Study of Classical Be stars in open clusters older than 100 Myr
• The B and Be States of the Star EM Cepheus
• Asteroseismic Imprints of Mass Transfer in Binary Stars: Probing the Interiors of Donors and Accretors with Gravity and Acoustic Modes
• Spectroscopy of close visual binary components of the stable shell star 1 Del
• NGC 1850 BH1 is another stripped-star binary masquerading as a black hole
• Characterising the orbit and circumstellar environment of the high-mass binary MWC 166 A
• Irregular emission cycles in the Oe star HD 60848
• Revealing the Binarity of HD 36030 -- One of the Hottest Flare Stars
• Optical/IR studies of Be stars in NGC 6834 with emphasis on two specific stars
• Disc breaking through forced eccentricity growth
• Asteroseismology
• How many VHE gamma-ray binaries with young pulsars can be observed?
• Halo Photometry and Asteroseismology for 98 of the Brightest Stars Observed by TESS
• A Lagrangian construction of rotating stars
• Classical Be Stars and Classical Be Star Binaries from LAMOST DR12
• From gas to stars: MUSEings on the internal evolution of IC 1613
• Hydrodynamical models of the $\beta$ Lyr A circumstellar disc
• The impact of stellar rotations and binaries on the shape of upper main sequence near turn off in open cluster NGC\,6067
• Toward a Comprehensive Grid of Cepheid Models with MESA. IV. Modest Effects of Rotation on Blue Loops
• Investigation of projected rotational velocities of Be-type stars in LAMOST DR7
• A Catalog of 45,206 Hα Emission-line Stars from LAMOST MRS DR7
• Identification and spectroscopic characterization of 128 new Herbig stars
• Long-term optical variability of high-mass X-ray binaries. III. Polarimetry
• Population synthesis of neutron star X-ray binaries associated with supernova remnants
• The identification of new Herbig Ae/Be stars from LAMOST DR7
• Fast optical variability in Supergiant X-ray binaries
• Revisiting Viscous Transonic Decretion Disks of Be Stars