Star formation and evolution in accretion disks around massive black holes- Star formation and evolution in accretion disks

We develop an exploratory model for the outer, gravitationally unstable reg ions of accretion disks around massive black holes. We consider black holes of mass 10(6) to 10(10) M., and primeval or solar abundances. In a first s tep we study star formation and evolution in a purely gaseous marginally un stable disk, and we show that unstable fragments should collapse rapidly an d give rise to compact objects (planets or protostars), which then accrete at a high rate and in less than 106 pears acquire a mass of a few tens of M ., according to a mechanism first proposed by Artymowicz et al. (1993). Whe n these stars explode as supernovae, the supernova shells break out of the disk, producing strong outflows. We show that the gaseous disk is able to s upport a large number of massive stars and supernovae while staying relativ ely homogeneous. An interesting aspect is that the residual neutron stars c an undergo other accretion phases, leading to other (presumably powerful) s upernova explosions. In a second step we assume that the regions at the per iphery of the disk provide a quasi stationary mass inflow during the lifeti me of quasars or of their progenitors, i.e. similar to 10(8) yrs, and that the whole mass transport is ensured by the supernovae, which induce a trans fer of angular momentum towards the exterior, as shown by the numerical sim ulations of Rozyczka et al. (1995). Assuming that the star formation rate i s proportional to the growth rate of the gravitational instability, we solv e the disk structure and determine the gas and the stellar densities, the h eating being provided mainly by the stars themselves. We find self-consiste nt solutions in which the gas is maintained in a state very close to gravit ational instability, in a ring located between 0.1 and 10 pc for a black ho le mass of 10(6)M., and between 1 and 100 pc for a black hole mass of 10(8) M. or larger, whatever the abundances, and for relatively low accretion rat es (less than or equal to 10% of the critical accretion rate). For larger a ccretion rates the number of stars becomes so large that they inhibit any f urther star formation, and/or the rate of supernovae is so high that they d istroy the homogeneity and the marginal stability of the disk. We postpone the study of this case. Several consequences of this model can be envisioned, besides the fact that it proposes a solution to the problem of the mass transport in the interme diate region of the disk when global instabilities do not work. As a first consequence, it could explain the high velocity metal enriched outflows imp lied by the presence of the broad absorption lines in quasars. As a second consequence it could account for a pregalactic enrichment of the intergalac tic medium, if black holes formed early in the Universe. Finally it could p rovide a triggering mechanism for starbursts in the central regions of gala xies. A check of the model would be to detect a supernova exploding within a few parsecs from the center of an AGN, an observation which can be perfor med in the near future.