1,488 research outputs found
The chemical evolution of Barium and Europium in the Milky Way
We compute the evolution of the abundances of barium and europium in the
Milky Way and we compare our results with the observed abundances from the
recent UVES Large Program "First Stars". We use a chemical evolution model
which already reproduces the majority of observational constraints. We confirm
that barium is a neutron capture element mainly produced in the low mass AGB
stars during the thermal-pulsing phase by the 13C neutron source, in a slow
neutron capture process. However, in order to reproduce the [Ba/Fe] vs. [Fe/H]
as well as the Ba solar abundance, we suggest that Ba should be also produced
as an r-process element by massive stars in the range 10-30 solar masses. On
the other hand, europium should be only an r-process element produced in the
same range of masses (10-30 solar masses), at variance with previous
suggestions indicating a smaller mass range for the Eu producers. As it is well
known, there is a large spread in the [Ba/Fe] and [Eu/Fe] ratios at low
metallicities, although smaller in the newest data. With our model we estimate
for both elements (Ba and Eu) the ranges for the r-process yields from massive
stars which better reproduce the trend of the data. We find that with the same
yields which are able to explain the observed trends, the large spread in the
[Ba/Fe] and [Eu/Fe] ratios cannot be explained even in the context of an
inhomogeneous models for the chemical evolution of our Galaxy. We therefore
derive the amount by which the yields should be modified to fully account for
the observed spread. We then discuss several possibilities to explain the size
of the spread. We finally suggest that the production ratio of [Ba/Eu] could be
almost constant in the massive stars.Comment: 14 pages, 17 figures, accepted for pubblication in A&
Loss of star forming gas in SDSS galaxies
Using the star formation rates from the SDSS galaxy sample, extracted using
the MOPED algorithm, and the empirical Kennicutt law relating star formation
rate to gas density, we calculate the time evolution of the gas fraction as a
function of the present stellar mass. We show how the gas-to-stars ratio varies
with stellar mass, finding good agreement with previous results for smaller
samples at the present epoch. For the first time we show clear evidence for
progressive gas loss with cosmic epoch, especially in low-mass systems. We find
that galaxies with small stellar masses have lost almost all of their cold
baryons over time, whereas the most massive galaxies have lost little. Our
results also show that the most massive galaxies have evolved faster and turned
most of their gas into stars at an early time, thus strongly supporting a
downsizing scenario for galaxy evolution.Comment: 29 pages, 9 figures, ApJ, accepte
The N/O Plateau of Blue Compact Galaxies: Monte Carlo Simulations of the Observed Scatter
Chemical evolution models and Monte Carlo simulation techniques have been
combined for the first time to study the distribution of blue compact galaxies
on the N/O plateau. Each simulation comprises 70 individual chemical evolution
models. For each model, input parameters relating to a galaxy's star formation
history (bursting or continuous star formation, star formation efficiency),
galaxy age, and outflow rate are chosen randomly from ranges predetermined to
be relevant. Predicted abundance ratios from each simulation are collectively
overplotted onto the data to test its viability. We present our results both
with and without observational scatter applied to the model points. Our study
shows that most trial combinations of input parameters, including a simulation
comprising only simple models with instantaneous recycling, are successful in
reproducing the observed morphology of the N/O plateau once observational
scatter is added. Therefore simulations which include delay of nitrogen
injection are no longer favored over those which propose that most nitrogen is
produced by massive stars, if only the plateau morphology is used as the
principal constraint. The one scenario which clearly cannot explain plateau
morphology is one in which galaxy ages are allowed to range below 250 Myr. We
conclude that the present data for the N/O plateau are insufficient by
themselves for identifying the portion of the stellar mass spectrum most
responsible for cosmic nitrogen production.Comment: 41 pages, 15 figures; accepted by ApJ, to appear Aug. 20, 200
On Dwarf Galaxies as the Source of Intracluster Gas
Recent observational evidence for steep dwarf galaxy luminosity functions in
several rich clusters has led to speculation that their precursors may be the
source of the majority of gas and metals inferred from intracluster medium
(ICM) x-ray observations. Their deposition into the ICM is presumed to occur
through early supernovae-driven winds, the resultant systems reflecting the
photometric and chemical properties of the low luminosity dwarf spheroidals and
ellipticals we observe locally. We consider this scenario, utilising a
self-consistent model for spheroidal photo-chemical evolution and gas ejection
via galactic superwinds. Insisting that post-wind dwarfs obey the observed
colour-luminosity-metallicity relations, we conclude that the bulk of the ICM
gas and metals does not originate within their precursors.Comment: 43 pages, 8 figures, LaTeX, also available at
http://msowww.anu.edu.au/~gibson/publications.html, to appear in ApJ, Vol
473, 1997, in pres
The mass surface density in the local disk and the chemical evolution of the Galaxy
We have studied the effect of adopting different values of the total baryonic
mass surface density in the local disk at the present time in a model for the
chemical evolution of the Galaxy. We have compared our model results with the
G-dwarf metallicity distribution, the amounts of gas, stars, stellar remnants,
infall rate and SN rate in the solar vicinity, and with the radial abundance
gradients and gas distribution in the disk. This comparison strongly suggests
that the value of the total baryonic mass surface density in the local disk
which best fits the observational properties should lie in the range 50-75 Msun
pc-2, and that values outside this range should be ruled out.Comment: 6 pages, LaTeX, 3 figures, accepted for publication in the
Astrophysical Journal, uses emulateapj.st
Evidence of a Metal Rich Galactic Bar from the Vertex Deviation of the Velocity Ellipsoid
We combine radial velocities, proper motions, and low resolution abundances
for a sample of 315 K and M giants in the Baade's Window (l,b)=(0.9,-4)
Galactic bulge field. The velocity ellipsoid of stars with [Fe/H]>-0.5 dex
shows a vertex deviation in the plot of radial versus transverse velocity,
consistent with that expected from a population with orbits supporting a bar.
We demonstrate that the significance of this vertex deviation using
non-parametric rank correlation statistic is >99%. The velocity ellipsoid for
the metal poor ([FeH]<-0.5) part of the population shows no vertex deviation
and is consistent with an isotropic, oblate rotating population. We find no
evidence for kinematic subgroups, but there is a mild tendency for the vertical
velocity dispersion sigma_b to decrease with increasing metallicity.Comment: 4 pages, ApJ Letters, submitte
Formation & evolution of the Galactic bulge: constraints from stellar abundances
We compute the chemical evolution of the Galactic bulge in the context of an
inside-out model for the formation of the Milky Way. The model contains updated
stellar yields from massive stars. The main purpose of the paper is to compare
the predictions of this model with new observations of chemical abundance
ratios and metallicity distributions in order to put constraints on the
formation and evolution of the bulge. We computed the evolution of several
alpha-elements and Fe and performed several tests by varying different
parameters such as star formation efficiency, slope of the initial mass
function and infall timescale. We also tested the effect of adopting a primary
nitrogen contribution from massive stars. The [alpha/Fe] abundance ratios in
the Bulge are predicted to be supersolar for a very large range in [Fe/H], each
element having a different slope. These predictions are in very good agreement
with most recent accurate abundance determinations. We also find a good fit of
the most recent Bulge stellar metallicity distributions. We conclude that the
Bulge formed on a very short timescale (even though timescales much shorter
than about 0.1 Gyr are excluded) with a quite high star formation efficiency of
about 20 Gyr and with an initial mass function more skewed toward high
masses (i.e. x <= 0.95) than the solar neighbourhood and rest of the disk. The
results obtained here are more robust than previous ones since they are based
on very accurate abundance measurements.Comment: 26 pages, 9 figures, accepted for publication in A&
The Earliest Phases of Galaxy Evolution
In this paper we study the very early phases of the evolution of our Galaxy
by means of a chemical evolution model which reproduces most of the
observational constraints in the solar vicinity and in the disk. We have
restricted our analysis to the solar neighborhood and present the predicted
abundances of several elements (C, N, O, Mg, Si, S, Ca, Fe) over an extended
range of metallicities to compared to previous
models. We adopted the most recent yield calculations for massive stars taken
from different authors (Woosley & Weaver 1995 and Thielemann et al. 1996) and
compared the results with a very large sample of data, one of the largest ever
used to this purpose. These data have been analysed with a new and powerful
statistical method which allows us to quantify the observational spread in
measured elemental abundances and obtain a more meaningful comparison with the
predictions from our chemical evolution model. Our analysis shows that the
``plateau'' observed for the [/Fe] ratios at low metallicities () is not perfectly constant but it shows a slope, especially for
oxygen. This slope is very well reproduced by our model with both sets of
yields. This is not surprising since realistic chemical evolution models,
taking into account in detail stellar lifetimes, never predicted a completely
flat plateau. This is due either to the fact that massive stars of different
mass produce a slightly different O/Fe ratio or to the often forgotten fact
that supernovae of type Ia, originating from white dwarfs, start appearing
already at a galactic age of 30 million years and reach their maximum at 1 Gyr.Comment: 32 pages, 9 figures, to be published in Ap
Stellar populations in gas-rich galaxy mergers II. Feedback effects of Type Ia and II supernovae
We numerically investigate chemodynamical evolution of major disk-disk galaxy
mergers in order to explore the origin of mass-dependent chemical, photometric,
and spectroscopic properties observed in elliptical galaxies. We particularly
investigate the dependence of the fundamental properties on merger progenitor
disk mass (M_d). Main results obtained in this study are the following three.
(1) More massive (luminous) ellipticals formed by galaxy mergers between more
massive spirals have larger metallicity (Z) and thus show redder colors: The
typical metallicity ranges from ~ 1.0 solar abundance (Z~ 0.02) for ellipticals
formed by mergers with M_d = 10^10 M_solar to ~ 2.0 solar (Z ~ 0.04) for those
with M_d = 10^12 M_solar. (2) The absolute magnitude of negative metallicity
gradients developed in galaxy mergers is more likely to be larger for massive
ellipticals.
Absolute magnitude of metallicity gradient correlates with that of age
gradient in ellipticals in the sense that an elliptical with steeper negative
metallicity gradient is more likely to show steeper age gradient. (3) Both
M/L_B and M/L_K, where M, L_B, and L_K are total stellar mass of galaxy
mergers, B-band luminosity, and K-band one, respectively, depend on galactic
mass in such a way that more massive ellipticals have larger M/L_B and smaller
M/L_K.Comment: 58 pages 16 figures, ApJ in press (March 1999 issue
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