Volume 573, January 2015
|Number of page(s)||7|
|Section||Stellar structure and evolution|
|Published online||09 December 2014|
Magnetorotational instability in decretion disks of critically rotating stars and the outer structure of Be and Be/X-ray disks
Department of Theoretical Physics and AstrophysicsMasaryk
Received: 27 August 2014
Accepted: 17 October 2014
Context. Evolutionary models of fast-rotating stars show that the stellar rotational velocity may approach the critical speed. Critically rotating stars cannot spin up more, therefore they lose their excess angular momentum through an equatorial outflowing disk. The radial extension of such disks is unknown, partly because we lack information about the radial variations of the viscosity.
Aims. We study the magnetorotational instability, which is considered to be the origin of anomalous viscosity in outflowing disks.
Methods. We used analytic calculations to study the stability of outflowing disks submerged in the magnetic field.
Results. The magnetorotational instability develops close to the star if the plasma parameter is large enough. At large radii the instability disappears in the region where the disk orbital velocity is roughly equal to the sound speed.
Conclusions. The magnetorotational instability is a plausible source of anomalous viscosity in outflowing disks. This is also true in the region where the disk radial velocity approaches the sound speed. The disk sonic radius can therefore be roughly considered as an effective outer disk radius, although disk material may escape from the star to the insterstellar medium. The radial profile of the angular momentum-loss rate already flattens there, consequently, the disk mass-loss rate can be calculated with the sonic radius as the effective disk outer radius. We discuss a possible observation determination of the outer disk radius by using Be and Be/X-ray binaries.
Key words: stars: mass-loss / stars: evolution / stars: rotation / hydrodynamics / magnetohydrodynamics (MHD)
© ESO, 2014
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.