Chemical evolution with inhibited star formation rate

In this paper a simple model of chemical evolution is proposed, according to which active phases of star formation occured, each followed by a quiescent phase where the formation of stars withm1m was totally inhibited. The contraction of the gaseous component of the system was not stopped during quiescent phases, according to observed distributions of chemical and dynamical parameters in objects of different populations. Building up a detailed model we assume that: (i) star formation is inhibited during a given active phase, at a rate which is connected to the number of supernovae born at that time; (ii) total mass is conserved in the unit comoving volume; (iii) instantaneous recycling approximation holds. According to these hypothesis, theoretical metal abundance distributions as function of a single parameter may be derived and then compared with empirical distributions related to different kinds of populations. It is found that theoretical curves systematically overestimate the objects of lower metallicity and underestimate the objects of higher metallicity in respect to observations, as a possible consequence of the last two assumptions reported above. If we leave the detailed explanation of this effect to more refined models and taking other observational data into account, the chemical evolution of both Galactic spheroid component and solar neighbourhood may be derived and the results are to be considered as essentially qualitative. If three active phases of star formation occurred during the history of the Galactic spheroid component, the yield of metals, , had to increase passing from an active phase to the next even if (metal content) and the disk mass fraction,R _D, had not to exceed a value of about 0.4 when is assumed as a characteristic value for the disk component and the main part of this last is thought to have formed at the end of the second active phase. If a single active (and no quiescent) phase occurred during the history of the solar neighbourhood, the yield did not increase appreciably up to to-day and no more than half (if any) of the initial mass could escape from the system in form of galactic wind. A disk mass fractionR _D0.2 is further derived, if a birth-rate stellar function smoothly dependent on the gas mass fraction and a ratio of mean past to present star formation rate of the order of unity are to be reproduced, in agreement with recent observations.