for the formation of the disk in the solar region is also different from the timescale assumed for assembling the dwarf irregulars (i.e. $\tau=0.5 $Gyr), namely is $\tau=3$ Gyr. This choice ensures that the majority of the observational constraints in the solar neighbourhood are reproduced. The nucleosynthesis prescriptions adopted for the solar neighbourhood are exactly the same as for the dwarfs.

Results
=======

Besides Model 1 and 5 of Kunth et al. (1995) (model 1 and 3 respectively, in Table 3), we run several models by varying star formation efficiency and/or wind efficiency, all the other parameters being left the same as in Model 5 of Kunth et al. The model parameters are presented in Table 3. In column 1 are the model numbers and in column 2 the nucleosynthesis prescriptions. In particular, “STANDARD” refers to the prescriptions described in section III, whereas “N PRIMARY” refers to the assumption of primary production of N in massive stars, leaving all the rest unchanged.

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  ----- ------------- ------ -------------- --------- ----------
   MOD       yields   wind   $\nu$           $N_{B}$  duration
                             ($Gyr^{-1}$)             (Myrs)
    1   STANDARD      80     10                 1     50
    2   N PRIMARY     700    50                 1     20
    3   N PRIMARY     80     50                 1     20
    4   N PRIMARY     