star formation processes as in Gerola et al. (1980) and Matteucci and Tosi (1985), where a detailed description can be found.

The galactic wind is assumed to be simply proportional to the star formation rate (namely to the rate of explosion of type II SNe). This is a reasonable choice since the duration of the burst is so short that it does not allow the explosion of type Ia SNe, which occur all after the burst. The simplicity of the treatment of the galactic wind avoids also the introduction of other unknown parameters such as the efficiency of energy tranfer from stars and supernovae to the ISM. The rate of mass loss via a galactic wind is defined as follows:

$$\dot G_{iw}(t)\,=\, -w_{i}\psi(t)X_{iw}(t)$$ where $X_{iw}(t)=X_{i}(t)$ and $w_{i}$ is a free parameter containing all the information about the energy released by SNe and the efficiency with which such energy is transformed into gas escape velocity (note that Pilyugin (1992;1993) defines a wind parameter which is the inverse of $w_{i}$, namely the ratio of the star formation rate to the wind rate). The value of w$_{i}$ has been assumed to be different for different elements. In particular, the assumption has been made 