  We study a model of an accretion disc surrounded by an extended corona of the
temperature 10^7 - 10^8 K. This corona modifies the disk spectrum since it
redirects a significant fraction of the emission from the central parts towards
the more distant parts of the disk. The same corona is indirectly the source of
the broad emission lines because we expect the formation of cool clouds at the
basis of the corona due to thermal instabilities. We model the number of the
clouds and their motion through the corona adopting a few different physically
sound assumptions. Comparing the predicted optical/UV continua and emission
line ratios and profiles with the observed values we favor a particular model
of a typical quasar. It radiates at ~0.5 of the Eddington luminosity, the
corona surrounding the disk is additionally heated in excess of the Inverse
Compton heating, and the broad line clouds are most probably destroyed when
accelerated vertically above the sound speed within the corona.

Astrophysics