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 R_c of less than a pc, core mass M_core 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:

• Bloated stars, a phenomenon that makes mass loss more efficient (Edwards 1980, Alexander & Netzer 1994, 1997),
• Tidal disruption of stars (Hills 1975, Frank & Rees 1976),
• Star Collisions (Spitzer & Saslaw 1966, Colgate 1967, Courvoisier et al 1996, Rauch 1999).

To quantify all these processes, it is important now to define the corresponding characteristic radii.