Time evolution of the gas volume density in three models with α = 0.5. The l.h.s. column presents the time evolution in model 2, already shown in Fig. 4. The middle column shows model 4, in which the motions of stars are artificially suppressed. In the r.h.s. column we show model 5, for which the initial equilibrium distribution was constructed without taking gas self-gravity into account. Note the different evolutionary times in the latter model.