Oxygen, carbon and nitrogen evolution in galaxies

We discuss the evolution of oxygen, carbon and nitrogen in galaxies of different morphological type by adopting detailed chemical evolution models with different star formation histories (continuous star formation or starbursts). In all the models detailed nucleosynthesis prescriptions from supernovae of all types and low- and intermediate-mass stars are taken into account. We start by computing chemical evolution models for the Milky Way with different stellar nucleosynthesis prescriptions. Then, a comparison between model results and 'key' observational constraints allows us to choose the best set of stellar yields. Once the best set of yields is identified for the Milky Way, we apply the same nucleosynthesis prescriptions to other spirals (in particular M101) and dwarf irregular galaxies. We compare our model predictions with the [C,N,O/Fe] versus [Fe/H], log(C/O) versus 12 + log(O/H), log(N/O) versus 12+ log(O/H) and [C/O] versus [Fe/H] relations observed in the solar vicinity and draw the following conclusions. (i) There is no need to invoke strong stellar winds in massive stars in order to explain the evolution of the C/O ratio, as often claimed in the literature. (ii) The predicted [O/Fe] ratio as a function of metallicity is in very good agreement with the most recent data available for the solar vicinity, especially for halo stars. This fact again suggests that the oxygen stellar yields in massive stars computed by either Woosley & Weaver or Thielemann, Nomoto & Hashimoto without taking into account mass loss, reproduce the observations well. (iii) We predict that the gap observed in the [Fe/O] versus [O/H] at [O/H]similar to-0.3 dex should be observed also in C/O versus O/H. The existence of such a gap is predicted by our model for the Milky Way and is caused by a halt in the star formation between the end of the thick disc and the beginning of the thin disc phase. Such a halt is produced by the adopted threshold gas density for the star formation rate. (iv) This threshold is also responsible for the prediction of a very slow chemical enrichment between the time of formation of the solar system (4.5 Gyr ago) and the present time, in agreement with new abundance measurements. (v) The chemical evolution models for dwarf irregulars and spirals, adopting the same nucleosynthesis prescriptions of the best model for the solar neighbourhood, well reproduce the available constraints for these objects. (vi) By taking into account the results obtained for all the studied galaxies (Milky Way, M101, dwarf galaxies and DLAs) we conclude that there is no need for claiming a strong primary component of N produced in massive stars (M > 10 M-circle dot ). (vii) Moreover, there is a strong indication that C and N are mainly produced in low- and intermediate-mass stars, at variance with recent suggestions that most of the C should come from massive stars. In particular, intermediate-mass stars with masses between 4 and 8 M-circle dot contribute mostly to N (both primary and secondary) whereas those with masses between 1 and 3 M-circle dot contribute mostly to C. At the same time, our results suggest that the C yields computed for massive stars, without taking into account stellar rotation, are underestimated and should be at least a factor of 3 larger than the current values.