Rotation and Properties of Be Stars

Abstract

The actual characteristic average surface angular velocity ratio of the Be phenomenon may be higher than Ω/Ωc= 0.8 ± 0.1 derived fromvsinivalues not corrected from gravity darkening effects. Late Be Stars need not only higher values of Ω/Ωc, but also larger main sequence age ratios τ/τMSthan early Be Stars to display the Be phenomenon. Galactic field Be Stars appear at any MS phase, but their frequency is higher for τ/τMS≳ 0.5. Be Stars are probably neat differential rotators, i.e. they rotate with energy ratios kinetic/|gravitational| larger than withstood by critical rigid rotators. Initial formation conditions must favor the existence of fast rotators, while only a small fraction of them could be accounted for by mass transfer in close binary systems. Bn stars could perhaps represent a pre-Be phase.