= -\psi(t)(1+w_{i}) X_i(t)+$$ $$\int_{M_{L}}^{M_{Bm}}{\psi(t-\tau_m) 
Q_{mi}(t-\tau_m)\phi(m)dm}+$$ $$A\int_{M_{Bm}}^{M_{BM}}{\phi(m)}\bigl[\int_{\mu_{min}}
^{0.5}{f(\mu)\psi(t-\tau_{m1}) 
Q_{mi}(t-\tau_{m2})d\mu\bigr] dm}+$$ $$(1-A)\int_{M_{Bm}}^
{M_{BM}}{\psi(t-\tau_{m})Q_{mi}(t-\tau_m)\phi(m)dm}+$$ $$\int_{M_{BM}}^{M_U}{\psi(t-\tau_m)Q_{mi}(t-\tau_m)+ 
\phi(m)dm} + \dot  G_{i}(t)_{inf} 
\eqno(1)$$ where $G_i(t)$=$M_{gas}(t)X_i(t)/M_{tot}(t_{G})$ is the mass density of gas in the form of an element [*i*]{} normalized to the total mass at the present time $t_{G}$. The quantity $X_i(t) = G_i(t)/G(t)$ represents the abundance by mass of the element $i$ and by definition the summation over all the abundances of the elements present in the gas mixture is equal to unity. The quantity $G(t) = M_{gas}/M_{tot}$ is the total fractionary mass of gas, and $M_{tot}$ refers only to the mass present in the form of gas in the star forming region. The possible presence of a dark matter halo is not considered here, given the simple treatment of the development of a galactic wind, as we will see in the following.

The star formation rate we assume during the burst, $\psi(t)$, is defined as: $$\psi(t)\,=-\nu \, \eta(t) \, G(t)$$

where $\nu$ is the star formation efficiency (expressed in units of Gyr$^{-1}$), and represents the inverse of the timescale of star formation, namely the timescale necessary to consume all the gas in the star forming region; $\eta(t)$ takes into account the stochastic nature of the 