139 research outputs found
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
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
Chemical behavior of the Dwarf Irregular Galaxy NGC 6822. Its PN and HII region abundances
We aim to derive the chemical behavior of a significant sample of PNe and HII
regions in the irregular galaxy NGC 6822 The selected objects are distributed
in different zones of the galaxy. Due to the faintness of PNe and HII regions
in NGC 6822, to gather spectroscopic data with large telescopes is necessary.
We obtained a well suited sample of spectra by employing VLT-FORS 2 and
Gemini-GMOS spectrographs. Ionic and total abundances are calculated for the
objects where electron temperatures can be determined through the detection of
[OIII] \lambda 4363 or/and [NII] \lambda 5755 lines. A "simple" chemical
evolution model has been developed and the observed data are used to compute a
model for NGC 6822 in order to infer a preliminary chemical history in this
galaxy. Confident determinations of He, O, N, Ne, S and Ar abundances were
derived for a sample of 11 PNe and one HII region. We confirm that the present
ISM is chemically homogeneous, at least in the central 2 kpc of the galaxy,
showing a value 12+log O/H = 8.060.04. From the abundance pattern of PNe,
we identified two populations: a group of young PNe with abundances similar to
HII regions and a group of older objects with abundances a factor of two lower.
A couple of extreme Type I PNe were found. No third dredge-up O enrichement was
detected in PNe of this galaxy. The abundance determinations allow us to
discuss the chemical behavior of the present and past ISM in NGC 6822. Our
preliminary chemical evolution model predicts that an important gas-mass lost
occurred during the first 5.3 Gyr, that no star higher than 40 M was
formed, and that 1% of all 3-15 M stars became binary systems
progenitors to SNIa.Comment: 15 pages, 3 figures and 4 tables. A&A, Accepted 13/06/200
Carbon and oxygen abundances from recombination lines in low-metallicity star-forming galaxies. Implications for chemical evolution
We present deep echelle spectrophotometry of the brightest emission-line
knots of the star-forming galaxies He 2-10, Mkn 1271, NGC 3125, NGC 5408, POX
4, SDSS J1253-0312, Tol 1457-262, Tol 1924-416 and the HII region Hubble V in
the Local Group dwarf irregular galaxy NGC 6822. The data have been taken with
the Very Large Telescope Ultraviolet-Visual Echelle Spectrograph in the
3100-10420 {\AA} range. We determine electron densities and temperatures of the
ionized gas from several emission-line intensity ratios for all the objects. We
derive the ionic abundances of C and/or O from faint pure
recombination lines (RLs) in several of the objects, permitting to derive their
C/H and C/O ratios. We have explored the chemical evolution at low
metallicities analysing the C/O vs. O/H, C/O vs. N/O and C/N vs. O/H relations
for Galactic and extragalactic HII regions and comparing with results for halo
stars and DLAs. We find that HII regions in star-forming dwarf galaxies occupy
a different locus in the C/O vs. O/H diagram than those belonging to the inner
discs of spiral galaxies, indicating their different chemical evolution
histories, and that the bulk of C in the most metal-poor extragalactic HII
regions should have the same origin than in halo stars. The comparison between
the C/O ratios in HII regions and in stars of the Galactic thick and thin discs
seems to give arguments to support the merging scenario for the origin of the
Galactic thick disc. Finally, we find an apparent coupling between C and N
enrichment at the usual metallicities determined for HII regions and that this
coupling breaks in very low-metallicity objects.Comment: 27 pages, 12 figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
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