SLIM ACCRETION DISCS - A MODEL FOR ADAF-SLE TRANSITIONS

We numerically construct slim, global, vertically integrated models of optically thin, transonic accretion discs around black holes, assumin g a regularity condition at the sonic radius and boundary conditions a t the outer radius of the disc and near the black hole. In agreement w ith several previous studies, we find two branches of shock-free solut ions, in which the cooling is dominated either by advection or by loca l radiation. We also confirm that the part of the accretion how where advection dominates is in some circumstances Limited in size: it does not extend beyond a certain outer Limiting radius. New results found i n our paper concern the location of the limiting radius and the proper ties of the flow near to it. In particular, we find that beyond the li miting radius the advective-dominated solutions match on to Shapiro, L ightman & Eardley (SLE) discs through a smooth transition region. Ther efore, the full global solutions are shock-free and unlimited in size. There is no need to postulate an extra physical effect (e.g. evaporat ion) for triggering the ADAF-SLE transition. It occurs as a result of standard accretion processes described by the classic slim disc equati ons.