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Astron. Astrophys. 361, 175-188 (2000)


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Evaporation: The change from accretion via a thin disk to a coronal flow

F. Meyer 1, B.F. Liu 1,2 and E. Meyer-Hofmeister 1

1 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, 85740 Garching, Germany
2 Chinese Academy of Sciences, Yunnan Observatory, P.O.Box 110, Kunming 650011, P.R. China

Received 19 April 2000 / Accepted 21 June 2000

Abstract

We present a model for a corona above a geometrically thin standard disk around a black hole. This corona is fed by matter from the thin disk which evaporates from the cool layers underneath. An equilibrium establishes between the cool accretion stream and the hot flow. In the inner region evaporation becomes so efficient that at low accretion rates all matter flows via the corona and proceeds towards the black hole as an advection-dominated accretion flow (ADAF). We investigate this transition for accretion disks around stellar black holes. We derive a set of four ordinary differential equations which describe the relevant processes and solve them numerically. The evaporation efficiency has a maximum value where the advection-dominated structure of the corona (at large distances from the black hole) changes to a radiation-dominated structure (at small distances). This maximum can be related to the observed spectral transitions in black hole X-ray binaries. Our solutions of coronal structure are valid for both, accretion in galactic X-ray transients and in AGN.

Key words: accretion, accretion disks – black hole physics – X-rays: stars – stars: individual: Cygnus X-1, Nova Muscae 1991 – galaxies: nuclei

Send offprint requests to: Emmi Meyer-Hofmeister (emm@mpa-garching.mpg.de)

© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000

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