2.2. Are stars a possible fuel?
The stars themselves could provide gas to the nucleus, through their mass loss, if there is a local stellar cluster, dense and compact enough (core radius Rc of less than a pc, core mass Mcore of the order of 108 M). However, the mass loss rate derived from normal stellar evolution gives only 10-11 M/yr/M, orders of magnitude below the required rate of a few M /yr. The contribution will be significant, only if a massive stellar cluster (4 109 M) has just formed through a starburst (Norman & Scoville 1988). A coeval cluster can liberate 109 M on 108 yr, since mainly massive stars evolve together in the beginning. Thus the existence of a starburst in the first place solves also the problem of the AGN fueling, as in the symbiosis model of Williams et al (1999). The angular momentum problem is now reported to the starburst fueling, and could be solved only through large-scale dynamical processes.
There are several processes to fuel gas to the black hole, directly from the stars, and these could work for the low-luminosity end of AGNs; one can invoke:
To quantify all these processes, it is important now to define the corresponding characteristic radii.