THE MAIN TYPES OF STAR-FORMATION IN GALACTIC NUCLEI

We continue the analysis of star formation in a galactic nucleus by mo deling the dynamical evolution of the nuclear gas: Initially, a massiv e gas cloud of 10(8)M(.) and 200 pc diameter sits in the extended stel lar cluster of the nuclear bulge. The gas is supported against collaps e by turbulent motions. As the turbulent energy is dissipated the clou d radius shrinks. Above some critical gas density, star formation occu rs and the subsequent supernova explosions of type II can replenish th e turbulent energy. Besides the loss of gas through star formation, th ere is a constant supply of fresh gas from outside the nucleus. In a p revious paper we discussed repetitive bursts of star formation. Here w e extend the discussion to four other possible scenarios: a) catastrop hic collapse of the gas towards a very dense and massive stellar clust er with the subsequent formation of a supermassive star collapsing fin ally into a supermassive black hole, b) repetitive expulsions of the g as from the nucleus, c) quasi-stationarity, and d) violent mass influx into the nucleus initiated by collision with another galaxy. We prese nt calculations for the four new scenarios under realistic assumptions of the basic parameters of a galactic nucleus. Because of the simplic ity of our model, it cannot precisely limit the range of the parameter s of a galactic nucleus under which a particular kind of solution is r ealized, but only shaw their order of magnitude. The simplicity has, o n the other hand, the advantage that the numerical results can easily be understood and interpreted in a semi-quantitative way.