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

نویسندگان

  • Stanley P. Owocki
  • S. P. Owocki
چکیده

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.

برای دانلود متن کامل این مقاله و بیش از 32 میلیون مقاله دیگر ابتدا ثبت نام کنید

ثبت نام

اگر عضو سایت هستید لطفا وارد حساب کاربری خود شوید

منابع مشابه

Are jets rotating at the launching?

I argue that the Doppler shift asymmetries observed in some young stellar object (YSO) jets result from the interaction of the jets with the circumstellar gas, rather than from jets’ rotation. The jets do rotate, but at a velocity much below claimed values. During the meeting I carefully examined new claims, and found problems with the claimed jets’ rotation. I will challenge any future observa...

متن کامل

Formation of Semi-relativistic Jets from Magnetospheres of Accreting Neutron Stars: Injection of Hot Bubbles into a Magnetic Tower

We present the results of 2.5-dimensional resistive magnetohydrodynamic (MHD) simulations of the magnetic interaction between a weakly magnetized neutron star and its accretion disk. General relativistic effects are simulated by using the pseudo-Newtonian potential. We find that well-collimated jets traveling along the rotation axis of the disk are formed by the following mechanism: (1) The mag...

متن کامل

Magnetized Accretion - Ejection Structures : 2 . 5 D MHD simulations of continuous Ideal Jet launching from resistive accretion disks

We present numerical magnetohydrodynamic (MHD) simulations of a magnetized accretion disk launching trans-Alfvénic jets. These simulations, performed in a 2.5 dimensional time-dependent polytropic resistive MHD framework , model a resistive accretion disk threaded by an initial vertical magnetic field. The resistivity is only important inside the disk, and is prescribed as η = α m V A H exp(−2Z...

متن کامل

The Effect of Rotational Gravity Darkening on Magnetically Torqued Be Star Disks

In the magnetically torqued disk (MTD) model for hot star disks, as proposed and formulated by Cassinelli et al. (2002), stellar wind mass loss was taken to be uniform over the stellar surface. Here account is taken of the fact that as stellar spin rate So (= √ ΩoR 3/GM) is increased, and the stellar equator is gravity darkened, the equatorial mass flux and terminal speed are reduced, compared ...

متن کامل

Formation and Evolution of Very Low Mass Stars and Brown Dwarfs

The formation and evolution of brown dwarfs are currently ”hot topics” in cool star research. Latest observations and modeling efforts on disks, accretion, outflows, spatial distribution, and binarity in the context of the formation by ejection model and alternative scenarios were presented and discussed vigorously in this splinter session. A second major topic was rotation and activity of thes...

متن کامل

ذخیره در منابع من


  با ذخیره ی این منبع در منابع من، دسترسی به آن را برای استفاده های بعدی آسان تر کنید

برای دانلود متن کامل این مقاله و بیش از 32 میلیون مقاله دیگر ابتدا ثبت نام کنید

ثبت نام

اگر عضو سایت هستید لطفا وارد حساب کاربری خود شوید

عنوان ژورنال:

دوره   شماره 

صفحات  -

تاریخ انتشار 2003