325 research outputs found
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
and C III 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 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
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