Making a Be star: the role of rotation and pulsations

Abstract

Abstract. The Be phenomenon, i.e. the ejection of matter from Be stars into a circumstellar disk, has been a long lasting mystery. In the last few years, the CoRoT satellite brought clear evidence that Be outbursts are directly correlated to pulsations and rapid rotation. In particular the stochastic excitation of gravito-inertial modes, such as those detected by CoRoT in the hot Be star HD 51452, is enhanced thanks to rapid rotation. These waves increase the transport of angular momentum and help to bring the already rapid stellar rotation to its critical value at the surface, allowing the star to eject material. Below we summarize the recent observational and theoretical findings and describe the new picture of the Be phenomenon which arose from these results.

Fluid Dynamics Experiments for Planetary Interiors

“…During the sudden transition to the high-torque state, the velocity gradient across the TC is reduced. Sudden angular momentum outbursts, conceptually similar to the one described, occur also in astrophysical objects, e.g., the so-called Be-phenomenon where an almost critically spinning, type Be star ejects material that suddenly acquired too much angular momentum (Neiner and Mathis 2013). These outbursts might have a common origin with the ones observed in the 3-meter experiment.…”

An interferometric view of hot star disks

“…The theoretical foundations for these correlations are discussed by Neiner and Mathis (2014), with the transport of angular momentum from the core to the surface due to the κ mechanism applied for fast rotating stars. According to these authors, "the accumulation of angular momentum just below the surface of Be stars increases the surface velocity.…”

“…Other, more likely, is the presence of NRP frequencies occurring only in B stars that are similar to near-critical equatorial orbits, working as triggers to mass-loss episodes, as the proximity between Goss et al (2011) frequency F1 and F4 for Achernar suggests. If the κ mechanism if the origin of these pulsations and the transport of angular momentum as proposed by Neiner and Mathis (2014), the hotter temperatures or physical dimensions of O-type stars could completely block the mechanism. …”