This is an electronic version of an article published in Monthly Notices of the Royal Astronomical Society. Mollá, M., Vílchez, J.M., Gavilán, M. and A.I. Díaz. The nitrogen-to-oxygen evolution in galaxies: the role of the star formation rate. Monthly Notices of the Royal Astronomical Society 372 (2006): 1069-108

Abstract. We present a set of low and intermediate mass star yields based on a modeling of the TP-AGB phase which affects the production of nitrogen and carbon. These yields are evaluated by using them in a Galaxy Chemical Evolution model, with which we analyze the evolution of carbon abundances. By comparing the results with those obtained with other yield sets, and with a large amount of observational data, we conclude that the model using these yields combined with those from Woosley & Weaver (1995) for massive stars properly reproduce all the data. The model reproduces well the increase of C/O with increasing O/H abundances. Since these massive star yields do not include winds, it implies that these stellar winds might have a smoother dependence on metallicity than usually assumed and that a significant quantity of carbon proceeds from LIM stars.

We present a set of 144 galactic chemical evolution models applied to a Milky Way analogue, computed using four sets of low+intermediate star nucleosynthetic yields, six massive star yield compilations, and six functional forms for the initial mass function. The integrated or true yields for each combination are derived. A comparison is made between a grid of multiphase chemical evolution models computed with these yield combinations and empirical data drawn from the Milky Way's disc, including the solar neighbourhood. By means of a χ 2 methodology, applied to the results of these multiphase models, the best combination of stellar yields and initial mass function capable of reproducing these observations is identified.

This work studies the relation between gas-phase oxygen abundance and stellar-togas fraction in nearby galaxies. We first derive the theoretical prediction, and argue that this relation is fundamental, in the sense that it must be verified regardless of the details of the gas accretion and star formation histories. Moreover, it should hold on 'local' scales, i.e. in regions of the order of 1 kpc. These predictions are then compared with a set of spectroscopic observations, including both integrated and resolved data. Although the results depend somewhat on the adopted metallicity calibration, observed galaxies are consistent with the predicted relation, imposing tight constraints on the mass-loading factor of (enriched) galactic winds. The proposed parametrization of the star fraction-metallicity relation is able to describe the observed dependence of the oxygen abundance on gas mass at fixed stellar mass. However, the 'local' mass-metallicity relation also depends on the relation between stellar and gas surface densities.

Aims. We analyze the impact on the Galactic nitrogen abundances with a new set of low and intermediate mass star yields. These yields contain a significant yield of primary nitrogen from intermediate mass stars. Methods. We use these yields as an input for a galactic chemical evolution model and study the nitrogen abundances in the halo and in the disc, and compare them with models obtained using other yield sets and with a large amount of observational data. Results. We find that with these new yields our model reproduces the observed trends adequately. In particular, these yields solve the historical problem of nitrogen evolution, giving the right level of relative abundance N/O by producing a primary component in intermediate mass stars. Moreover, using different evolutionary rates for each radial region of the Galaxy, we can explain the observed N dispersion.