Rotation and Mass Ejection: the Launching of Be-Star Disks

The characteristic signature of Be stars is the Balmer line emission understood to arise in a circumstellar disk. Unlike the accretion disks of protostars or mass-exchange binary systems, the evolved and generally single or wide-binary status of Be stars seems to require that its disk must form from mass ejection (a.k.a. decretion) from the star itself. In this paper, I use analogies with launching orbital satellites to discuss two candidate processes (radiation, pulsation) for driving such orbital mass ejection, with particular emphasis on the role of the rapid, possibly near-critical, rotation of Be stars in facilitating the formation of their signature disks. 1. The Puzzle of Be Disks Within the context of this Symposium, an especially good laboratory to study the role of rapid stellar rotation is provided by the classical Be stars, which are among the most rapidly rotating massive stars. Be stars are main-sequence or subgiant spectral type B stars characterized by Balmer emission (e.g., H ) that originates in a circumstellar disk. Understanding the nature of such stars, and in particular the origin of their circumstellar disks, is one of the longest-standing challenges in astronomy (Secchi 1867; Slettebak and Snow 1987; Balona, Henrichs, and LeContel 1994; Smith, Henrichs, and Fabregat 1999). Disks are a common consequence in astrophysical accretion systems, wherein they form as a means to provide outward viscous transport of the angular momentum of the infalling material (Frank et al. 2002). Such accretion disks occur, for example, in protostellar nebulae, or in close binary systems with mass exchange. But Be stars are clearly too old to have retained a protostellar disk (indeed, in many Be stars the Balmer emission signatures of a disk are observed to come and go on timescales of months to decades), and moreover they are not generally found to be in close, mass-exchange binary systems. Thus lacking an outside source of material, it seems instead that Be disks must originate from ejection or decretion of mass from the underlying star. Identifying the speci c dynamical mechanisms for achieving this \inside-out" formation of a circumstellar disks is perhaps the central puzzle underlying the Be phenomenon.