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Keplerian accretion discs around massive black holes (MBHs) aregravitationally unstable beyond a few hundredths of parsec and should collapseto form stars. Indeed an accretion/star formation episode took place a fewmillions years ago in the Galactic Center (GC). This raises the question of howthe disc can survive in AGN and quasars and continue to transport mattertowards the black hole. We study the accretion/star formation process, with oneaim in mind, to show that a spectrum similar to the observed AGN one can beproduced by the disc. We compute models of stationary accretion discs, bothcontinuous and clumpy. Continuous discs must be maintained in a state ofmarginal stability for the rate of star formation to remain modest, so theyrequire additional heating and transport of angular momentum. Non-viscousheating can be provided by stellar illumination, but momentum transport bysupernovae is insufficient to sustain a marginal state, except at the veryperiphery of the disc. In clumpy discs it is possible to account for therequired accretion rate through interactions between clouds, but this model isunsatisfactory as its parameters are tightly constrained without any physicaljustification. Finally one must appeal to non-stationary discs withintermittent accretion episodes like those that occurred in the GC, but such amodel is probably not applicable to luminous high redshift quasars neither toradio-loud quasars.

Star formation in accretion discs : from the Galactic Center to Active Galactic Nuclei