PN-Carbon Yields and the Chemical Evolution of the Galaxy

Two sets of observational carbon stellar yields for low-and-intermediate mass stars are computed from planetary nebulae abundances derived from C II $\lambda4267$ and C III $\lambda\lambda1906+1909$ lines, respectively. By comparing C/O values observed in stars of the solar vicinity and Galactic HII regions with those predicted by chemical evolution models for the Galaxy, which assume these observational yields, I conclude that the C abundances derived from permitted lines are better than those derived from forbidden lines.Comment: 2 pages, Proceedings of the conference "Ionized Gaseous Nebulae", Nov 21-24, 2000; RevMexAA in pres

The delayed contribution of low and intermediate mass stars to chemical galactic enrichment: An analytical approach

We find a new analytical solution for the chemical evolution equations, taking into account the delayed contribution of all low and intermediate mass stars (LIMS) as one representative star that enriches the interstellar medium.This solution is built only for star formation rate proportional to the gas mass in a closed box model. We obtain increasing C/O and N/O ratios with increasing O/H, behavior impossible to match with the Instantaneous Recycling Approximation (IRA). Our results, obtained by two analytical equations, are very similar to those found by numerical models that consider the lifetimes of each star. This delayed model reproduces successfully the evolution of C/O-O/H and Y-O relations in the solar vicinity. This analytical approximation is a useful tool to study the chemical evolution of elements produced by LIMS when a galactic chemical evolutionary code is not available.Comment: 19 pages, 5 figures, to be published in the RevMexAA in October 200

HII Regions And the Protosolar Helium, Carbon, and Oxygen Abundances in the Context of Galactic Chemical Evolution

We present chemical evolution models of the Galactic disk with different Z-dependent yields. We find that a moderate mass loss rate for massive stars of solar metallicity produces an excellent fit to the observed C/H and C/O gradients of the Galactic disk. The best model also fits: the H, He, C, and O abundances derived from recombination lines of M17, the protosolar abundances,and the C/O-O/H, C/Fe-Fe/H, and O/Fe-Fe/H relations derived from solar vicinity stars. The agreement of the model with the protosolar abundances implies that the Sun originated at a galactocentric distance similar to the one it has. Our model for $r=3$ kpc implies that a fraction of the stars in the direction of the bulge formed in the inner disc. We obtain a good agreement between our model and the C/O versus O/H relationship derived from extragalactic H~{\sc ii} regions in spiral galaxies.Comment: 30 pages, 11 figures. Rev. Mex. Astron. Astrof. accepte

Chemical consequences of low star formation rates: stochastically sampling the IMF

When estimating the abundances which result from a given star formation event, it is customary to treat the IMF as a series of weight factors to be applied to the stellar yields, as a function of mass, implicitly assuming one is dealing with an infinite population. However, when the stellar population is small, the standard procedure would imply the inclusion of fractional numbers of stars at certain masses. We study the effects of small number statistics on the resulting abundances by performing an statistical sampling of the IMF to form a stellar population out of discrete numbers of stars. A chemical evolution code then follows the evolution of the population, and traces the resulting abundances. The process is repeated to obtain an statistical distribution of the resulting abundances and their evolution. We explore the manner in which different elements are affected, and how different abundances converge to the infinite population limit as the total mass increases. We include a discussion of our results in the context of dwarf spheroidal galaxies and show the recently reported internal dispersions in abundance ratios for dSph galaxies might be partly explained through the stochastic effects introduced by a low star formation rate, which can account for dispersions of over 2 dex in [C/O], [N/O], [C/Fe], [N/Fe] and [O/Fe].Comment: 13 pages, 13 figures, Accepted for publication in MNRA