Classical Be Stars

Abstract

Recent results for classical Be stars are reviewed and links to general astrophysics are presented. Classical Be stars are B-type stars close to the main sequence that exhibit line emission over the photospheric spectrum. The excess is attributed to a circumstellar gaseous component that is commonly accepted to be in the form of an equatorial disk. Since 1988, when the last such review was published, major progress has been made. The geometry and kinematics of the circumstellar environment can be best explained by a rotationally supported relatively thin disk with very little outflow, consistent with interferometric observations. The presence of shortterm periodic variability is restricted to the earlier type Be stars. This variation for at least some of these objects has been shown to be due to nonradial pulsation. For at least one star, evidence for a magnetic field has been observed. The mechanisms responsible for the production and dynamics of the circumstellar gas are still not constrained. Observations of nonradial pulsation beating phenomena connected to outbursts point toward a relevance of pulsation, but this mechanism cannot be generalized. Either the evidence that Be stars do not form a homogeneous group with respect to disk formation is growing or the short-term periodic variability is less important than previously thought. The statistics of Be stars investigated in open clusters of the Milky Way and the Magellanic Clouds has reopened the question of the evolutionary status of Be stars. The central B star is a fast rotator, although theoretical developments have revived the question of how high rotational rates are, so the commonly quoted mean value of about 70%-80% of the critical velocity may just be a lower limit. Be stars are in a unique position to make contributions to several important branches of stellar physics, e.g., asymmetric mass-loss processes, stellar angular momentum distribution evolution, astroseismology, and magnetic field evolution.

Multi-wavelength observations of the binary system PSR B1259−63/LS 2883 around the 2014 periastron passage

“…The asymmetry is sustained for at least ∼ 60 days, far longer than the expected 1-2 days required for disk material to circle the star. The asymmetry may indicate a slowly moving spiral density wave in the disk, typical of density waves observed in other Be stars (Porter & Rivinius 2003). The pulsar wind probably does not significantly alter the ionization levels in the circumstellar disk.…”

High-mass X-ray binaries in the Milky Way

“…By definition, Be stars are non super-giant B-type stars that have shown emission lines in their spectra, originating from a circumstellar disk expelled by a rapidly rotating star (Porter and Rivinius, 2003). A majority of these systems are transient sources exhibiting two type of outbursts.…”

Hubble Tarantula Treasury Project. V. The Star Cluster Hodge 301: The Old Face of 30 Doradus*

“…(2) Concerning the MSTO, even considering photometric errors, none of the models is able to reproduce the observed color spread in the magnitude range 16-18. This is not surprising given the presence of Be stars, which are thought to be fast rotators surrounded by an outflowing equatorial disk, likely causing their IR excess (see e.g., Porter & Rivinius 2003). (3) Only isochrones between 25 Myr and 30 Myr allow one to reproduce the MSTO luminosity (V∼ 16; corresponding to a mass ≈9 M e with PARSEC models), while the younger ones are at least 1 mag brighter than the MSTO tip.…”