STAR TRAPPING AND METALLICITY ENRICHMENT IN QUASARS AND ACTIVE GALACTIC NUCLEI

Recent observational evidence suggests that the metallicity in quasars within a wide range of redshifts, in particular in gas flowing out of the nuclear regions, may be approximately redshift-independent and comparable with or larger than solar. It is plausible that the nuclear metallicity can be internally generated and maintained at approximately time-stationary values in quasars. We identify and estimate efficiency of a mechanism for rapid metallicity enrichment of quasar nuclear gas (in general, in active galactic nuclei) based on star-gas interactions and equivalent to an unusual mode of massive star formation. The mechanism involves capture of low-mass stars from the host galaxy's nucleus by the assemblages of clouds or by accretion disks orbiting the central massive objects (e.g., black holes). Trapping of stars within gaseous disks/clouds occurs through resonant density and bending wave excitation, as well as by hydrodynamical drag, The time scale for trapping stars with total mass equal to that of disk fragment/cloud is of order Hubble time and is remarkably model-independent. The trapped stars, acting as seeds for ps accretion, allow for efficient conversion of nuclear ps, too hot to form stars via cloud collapse, into massive stars (up to approximately 10(2) M.). Such stars evolve rapidly toward the supernova stage (Type II, in the case of metal-poor stars). Supernovae ejecta provide heavy element contamination for both the disk gas orbiting the central black hole and for that part of ejecta which escapes from the gravitational potential of an active nucleus, Our results show that the described mechanism can produce features suggested by observations, for example, the (super) solar gas metallicity in the nucleus. Thus the observed metallicities in high-redshift quasars do not necessarily imply that global star formation and efficient chemical changes have occurred in their host galaxies at very early cosmological epochs. We concentrate on the first stages of the mechanism-trapping, accretion, and radial migration of the stars-and outline prospects for future work related to the population of supernova remnants in and around the disk.