[SEMINARI]: Dr. J. Gracia: The transition region between a standard disc and an advection dominated flow

[Arcadi Santamaria i Luna]

SEMINARI
LLOC: Seminari d'Astronomia i Astrofisica
DIA: 21 de gener de 2003 (21 January 2003)
HORA: 12 h.

       Dr. J. Gracia
       Landessternwarte Heidelberg, Germany
 
 " The transition region between a standard disc and 
 an advection dominated flow"
 
 
We study time-dependent models for rotating accretion flows onto black
holes, where a transition takes place from an outer cooling-dominated
disc to a radiatively inefficient flow in the inner region.  In order
to allow for a transition of this type we solve the energy equation,
both, for the gas and for the radiation field, including a radiative
cooling flux and a turbulent convective heat flux directed along the
negative entropy gradient.

The transition region is found to be highly variable, and a
corresponding variation expected for the associated total luminosity. 
In particular, rapid oscillations of the transition radius are present
on a timescale comparable with the local Keplerian rotation time. 
These oscillations are accompanied by a quasi-periodic modulation of
the mass flux at the outer edge of the advection-dominated accretion
flow (ADAF). We speculate about the relevance for the high-frequency
QPO phenomenon.