M. Gavilán

The Nitrogen-to-Oxygen evolution in galaxies: The role of the star formation rate

The main objective of the present work is to ckeck if the star formation effi ciency plays a relevant role in the evolution of the relative abundance N/O. In order to explore this idea, we analyze the evolution of the nitrogen-to-oxygen ratio as predicted by a set of computed theoretical models. These models consist of simulated galaxies with different total masses which are evolved assuming different collapse time scales and different star formation effi ciencies. The combinations of these two parameters produce different star formation histories, which in turn have, as we show, an important impact on the resulting N/O ratio. Since we want to check the effect of variations in these effi ciencies on our models results, the same stellar yield sets are used for all of them. The selected yields have an important contribution of primary nitrogen proceeding from low and intermediate mass stars, which implies that N is ejected with a certain delay with respect to O. It allows to obtain, as we demonstrate, a dispersion of results in the N/O-O/H plane when star formation effi ciencies vary which is in general agreement with observations. The model results for the N/O abundance ratio are in good agreement with most observational data trends. In particular, the beh avior shown by the extragalactic Hii regions is well reproduced with present time resulting abundances. Furthermore, the low N/O values estimated for high-redshift objects, such as those o btained for Damped Lyman Alpha (DLA) galaxies, as well as the higher (and constant) values of N/O observed for irregular and dwarf galaxies or halo stars, can be simultaneously obtained with our models at the same low oxygen abundances 12+ log(O/H)∼ 7. We therefore conclude that, even though there seems to be a general believe that abundance ratios depend mostly on stellar yields, these are not the only parameter at work when both elements are ejected by stars of different mass range, and that differences in the star formation history of galaxies and region s within them are a key factor to explain the data in the N/O-O/H plane.

Low and intermediate mass star yields: The evolution of carbon abundances

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.

Low and intermediate mass star yields. II. The evolution of nitrogen abundances

Aims. We analyze the impact on the Galactic nitrogen abundances of using a new set of low and intermediate mass star yields. These yields have a significant yield of primary nitrogen from intermedia te mass stars. Methods. We use these yields as an input to 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, using these new yields, our model adequately rep roduce the observed trends. In particular, these yields sol ve the historical problem of the evolution of nitrogen, giving the right level of relative abundance N/O by the production of a primary component in intermediate mass stars. Moreover, using different evolutionary rates in each radial region of the Galaxy, we may explain the observed N dispersion.

The chemical case for no winds in dwarf irregular galaxies

We argue that isolated gas-rich dwarf galaxies -- in particular, dwarf irregular (dIrr) galaxies -- do not necessarily undergo significant gas loss. Our aim is to investigate whether the observed properties of isolated, gas-rich dwarf galaxies, not affected by external environmental processes, can be reproduced by self-consistent chemo-photometric infall models with continuous star formation histories and no mass or metals loss. The model is characterized by the total mass of primordial gas available to the object, its characteristic collapse timescale, and a constant star formation efficiency. A grid of 144 such models has been computed by varying these parameters, and their predictions (elemental abundances, stellar and gas masses, photometric colors) have been compared with a set of observations of dIrr galaxies obtained from the literature. It is found that the models with moderate to low efficiency are able to reproduce most of the observational data, including the relative abundances of nitrogen and oxygen.

Formation and Evolution of Gas-Rich Dwarf Galaxies

We propose a scenario for the formation and evolution of isolated gas-rich dwarf galaxies based on the formation of stars fueled by continuous accretion of primordial gas from the surrounding intergalactic medium. Combining the relations between gas mass, nitrogen and oxygen abundances, B-band luminosity and (B-V) color, we conclude that smaller galaxies must form less stars per unit gas mass than bigger systems. Objects that show higher (N/O) than predicted by the models are good candidates for selective gas loss due to enriched galactic winds, whereas a low (N/O) ratio can be interpreted as evidence for a recent burst.

Chemical Evolution of CNO Abundances

New low and intermediate mass star yields recalculated by Buell (1987) are evaluated by using them in a Galactic Chemical Evolution model. We analyze their effects on CNO elemental abundances.