SIMBAD references

We study the chemical evolution of secondary and primary elements in a model which allows for inhomogeneities in the interstellar medium due to incomplete mixing of stellar ejecta, and a star formation localized in time and space. The average abundances are the same as the single value obtained from homogeneous models. The inhomogeneous star formation causes a scatter of the abundances of the ISM. Incomplete mixing has a strong influence on the relation between nitrogen and oxygen, giving rise to flatter N/O-O/H relations, thus mimicking a primary contribution to the nitrogen synthesis. Comparison with observational data from the Galaxy and external galaxies indicates that the probability of star formation is about T=0.1, in accord with the scatter of stellar metallicities which indicates that the degree of chemical inhomogeneity of the interstellar gas is indeed rather small.