Classical Be stars

By T. Rivinius, A. Carciofi, C. Martayan. The Astronomy and Astrophysics Review, 2013

692 citations recorded by the source corpus.

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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/40e5e260a30348b414907f22e6a371c892360480

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Citations in the corpus

The Zwicky Transient Facility: Science Objectives (2019, background): “One possible mechanism is the instability of the accretion disk (Rivinius et al. 2013).”
The Zwicky Transient Facility: System Overview, Performance, and First Results (2018, background): “A fraction of Be stars are known to exhibit photometric variability due to the non-radial pulsation, ejected material, stellar winds, or instability of the decretion disk (see review in Rivinius et al. 2013 and references therein).”
Gamma-ray binaries and related systems (2013, background): “In addition to this radiation-driven wind, Be stars have a dense equatorial outflow responsible for line emission in optical and excess continuum emission in infrared (Porter & Rivinius 2003).”
WIDE-FIELD INFRARED SURVEY EXPLORER OBSERVATIONS OF THE EVOLUTION OF MASSIVE STAR-FORMING REGIONS (2012, background): “Field Galactic AGB stars and Classical Be stars (CBe; Porter & Rivinius 2003) can also mimic the properties of young stars in surveys such as this one (Robitaille et al. 2008).”
Origin of the abundance patterns in Galactic globular clusters: constraints on dynamical and chemical properties of globular clusters (2007, background): “This is the kind of process believed to occur in Be stars (Porter & Rivinius 2003).”
Orbital parameters of the microquasar LS I +61 303 (2005, background): “Howarth 2004, Porter & Rivinius 2003). energy losses of relativistic electrons in the radio jets in the periastron vicinity (see Massi 2004b for details).”
Be/X-ray binaries (2011, background): “Be stars are non-supergiant fast-rotating B-type and luminosity class III-V stars which at some point of their lives have shown spectral lines in emission, hence the qualifier ”e” in their spectral types (Porter & Rivinius 2003; Balona 2000; Slettebak 1988).”
The MiMeS survey of Magnetism in Massive Stars : magnetic analysis of the O-type stars (2016, background): “…from other emission line stars and is attributed to a circumstellar decretion disk, usually considered to be a continuation of the Be phenomenon towards hotter spectral types (see Porter & Rivinius 2003; Rivinius et al. 2013, for further details of the emission characteristics of Be stars).”
Rotating massive main-sequence stars: II. Simulating a population of LMC early B-type stars as a test of rotational mixing (2011, background): “Commonly quoted required fractions of critical rotation for the occurrence of the Be phenomenon are 70–80% (Porter & Rivinius 2003), though values as low as ≥40% (Cranmer 2005) are also derived.”
High-mass X-ray binaries in the Milky Way (2013, background): “expelled by a rapidly rotating star (Porter and Rivinius, 2003).”
INTERNAL GRAVITY WAVES IN MASSIVE STARS: ANGULAR MOMENTUM TRANSPORT (2013, background): “There are many open questions with regard to this class of stars (see Porter & Rivinius (2003) for a review).”
A STELLAR ROTATION CENSUS OF B STARS: FROM ZAMS TO TAMS (2010, background): “However, Be stars are intrinsically variable in nature (Porter & Rivinius 2003), and in some cases (“transient Be stars”) the disk emission may even disappear at times (cf. McSwain et al. 2009).”
A CHARA ARRAY SURVEY OF CIRCUMSTELLAR DISKS AROUND NEARBY Be-TYPE STARS (2013, methodology): “Our survey provides a first epoch of diameter measurements that will serve in future studies of the growth and dissipation of disks that are frequently observed in Be stars (Porter & Rivinius 2003).”