Nitrogen and oxygen evolution in dwarf irregular galaxies

Abstract

Numerical models of chemical evolution have been computed for a sample of 40 dwarf irregular galaxies by assuming bursts of star formation and taking galactic winds into account. The evolution of nitrogen and oxygen has been followed up by means of the most recent results on stellar evolution. N and O abundances observed in these galaxies are well reproduced by theoretical models with the following implications: (i) a primary fraction of nitrogen ranging from 30 to 50 per cent can explain the N/O versus O diagrams of dwarf irregulars, as well as that of halo stars in our Galaxy; (ii) a Salpeter initial mass function is applicable to all these galaxies, while functions strongly varying with metallicity do not reproduce the observed data; (iii) as recently suggested, the upper limit of intermediate mass stars (i.e. stars igniting He non-degenerately, but developing an electron degenerate C–O core following the exhaustion of He at the centre)should be in the range 5–6 M⊙ instead of 8 M⊙ and varying from galaxy to galaxy; (iv) galactic winds with a mass loss rate ranging from 0 to approximately 30 times the star formation rate explain the observed spread in the chemical properties of dwarf irregulars. However it is not excluded that the gas loss rate could be even more than 30 times larger than the star formation rate, as seems to be required by IZw 18.