1,637 research outputs found
The Effects of radial inflow of gas and galactic fountains on the chemical evolution of M31
Galactic fountains and radial gas flows are very important ingredients in
modeling the chemical evolution of galactic disks. Our aim here is to study the
effects of galactic fountains and radial gas flows in the chemical evolution of
the disk of M31. We adopt a ballistic method to study the effects of galactic
fountains on the chemical enrichment of the M31 disk. We find that the landing
coordinate for the fountains in M31 is no more than 1 kpc from the starting
point, thus producing negligible effect on the chemical evolution of the disk.
We find that the delay time in the enrichment process due to fountains is no
longer than 100 Myr and this timescale also produces negligible effects on the
results. Then, we compute the chemical evolution of the M31 disk with radial
gas flows produced by the infall of extragalactic material and fountains. We
find that a moderate inside-out formation of the disk coupled with radial flows
of variable speed can very well reproduce the observed gradient. We discuss
also the effects of other parameters such a threshold in the gas density for
star formation and an efficiency of star formation varying with the galactic
radius. We conclude that the most important physical processes in creating disk
gradients are the inside-out formation and the radial gas flows. More data on
abundance gradients both locally and at high redshift are necessary to confirm
this conclusion.Comment: Accepted by 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
Chemical evolution of the Galactic Center
In recent years, the Galactic Center (GC) region (200 pc in radius) has been
studied in detail with spectroscopic stellar data as well as an estimate of the
ongoing star formation rate. The aims of this paper are to study the chemical
evolution of the GC region by means of a detailed chemical evolution model and
to compare the results with high resolution spectroscopic data in order to
impose constraints on the GC formation history.The chemical evolution model
assumes that the GC region formed by fast infall of gas and then follows the
evolution of alpha-elements and Fe. We test different initial mass functions
(IMFs), efficiencies of star formation and gas infall timescales. To reproduce
the currently observed star formation rate, we assume a late episode of star
formation triggered by gas infall/accretion. We find that, in order to
reproduce the [alpha/Fe] ratios as well as the metallicity distribution
function observed in GC stars, the GC region should have experienced a main
early strong burst of star formation, with a star formation efficiency as high
as 25 Gyr^{-1}, occurring on a timescale in the range 0.1-0.7 Gyr, in agreement
with previous models of the entire bulge. Although the small amount of data
prevents us from drawing firm conclusions, we suggest that the best IMF should
contain more massive stars than expected in the solar vicinity, and the last
episode of star formation, which lasted several hundred million years, should
have been triggered by a modest episode of gas infall/accretion, with a star
formation efficiency similar to that of the previous main star formation
episode. This last episode of star formation produces negligible effects on the
abundance patterns and can be due to accretion of gas induced by the bar. Our
results exclude an important infall event as a trigger for the last starburst.Comment: 10 pages, 8 figures, accepted for publication in MNRA
On the origin of the helium-rich population in the peculiar globular cluster Omega Centauri
In this contribution we discuss the origin of the extreme helium-rich stars
which inhabit the blue main sequence (bMS) of the Galactic globular cluster
Omega Centauri. In a scenario where the cluster is the surviving remnant of a
dwarf galaxy ingested by the Milky Way many Gyr ago, the peculiar chemical
composition of the bMS stars can be naturally explained by considering the
effects of strong differential galactic winds, which develop owing to multiple
supernova explosions in a shallow potential well.Comment: 2 pages, 1 figure, to appear in the Proceedings of IAU Symposium No.
268, Light Elements in the Universe (C. Charbonnel, M. Tosi, F. Primas, C.
Chiappini, eds., Cambridge Univ. Press
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&
On the typical timescale for the chemical enrichment from SNeIa in Galaxies
We calculate the type Ia supernova rate for different star formation
histories in galaxies by adopting the most popular and recent progenitor
models. We show that the timescale for the maximum in the type Ia supernova
rate, which corresponds also to time of the maximum enrichment, is not unique
but is a strong function of the adopted stellar lifetimes, initial mass
function and star formation rate. This timescale varies from Myr
for an instantaneous starburst to 0.3 Gyr for a typical elliptical
galaxy to Gyr for a disk of a spiral Galaxy like the Milky Way.
We also show that the typical timescale of 1 Gyr, often quoted as the typical
timescale for the type Ia supernovae, is just the time at which, in the solar
neighbourhood, the Fe production from supernovae Ia starts to become important
and not the time at which SNe Ia start to explode. As a cosequence of this, a
change in slope in the [O/Fe] ratio is expected in correspondance of this
timescale. We conclude that the suggested lack of supernovae Ia at low
metallicities produces results at variance with the observed [O/Fe] vs. [Fe/H]
relation in the solar region. We also compute the supernova Ia rates for
different galaxies as a function of redshift and predict an extended maximum
between redshift and for elliptical galaxies, and two
maxima, one at and the other at , for spiral galaxies,
under the assumption that galaxies start forming stars at and
, .Comment: 25 pages, 6 figures, accepted for pubblication from Ap
Chemical evolution of the bulge of M31: predictions about abundance ratios
We aim at reproducing the chemical evolution of the bulge of M31 by means of
a detailed chemical evolution model, including radial gas flows coming from the
disk. We study the impact of the initial mass function, the star formation rate
and the time scale for bulge formation on the metallicity distribution function
of stars. We compute several models of chemical evolution using the metallicity
distribution of dwarf stars as an observational constraint for the bulge of
M31. Then, by means of the model which best reproduces the metallicity
distribution function, we predict the [X/Fe] vs. [Fe/H] relations for several
chemical elements (O, Mg, Si, Ca, C, N). Our best model for the bulge of M31 is
obtained by means of a robust statistical method and assumes a Salpeter initial
mass function, a Schmidt-Kennicutt law for star formation with an exponent
k=1.5, an efficiency of star formation of , and an
infall timescale of Gyr. Our results suggest that the bulge
of M31 formed very quickly by means of an intense star formation rate and an
initial mass function flatter than in the solar vicinity but similar to that
inferred for the Milky Way bulge. The [/Fe] ratios in the stars of the
bulge of M31 should be high for most of the [Fe/H] range, as is observed in the
Milky Way bulge. These predictions await future data to be proven.Comment: Accepted for publication by MNRA
The cosmic dust rate across the Universe
We investigate the evolution of interstellar dust in the Universe by means of chemical evolution models of galaxies of different morphological types, reproducing the main observed features of present-day galaxies. We adopt the most updated prescriptions for dust production from supernovae and asymptotic giant branch stars as well as for dust accretion and destruction processes. Then, we study the cosmic dust rate in the framework of three different cosmological scenarios for galaxy formation: (i) a pure luminosity scenario, (ii) a number density evolution scenario, as suggested by the classical hierarchical clustering scenario and (iii) an alternative scenario, in which both spirals and ellipticals are allowed to evolve in number on an observationally motivated basis. Our results give predictions about the evolution of the dust content in different galaxies as well as the cosmic dust rate as a function of redshift. Concerning the cosmic dust rate, the best scenario is the alternative one, which predicts a peak at 2 < z < 3 and reproduces the cosmic star formation rate. We compute the evolution of the comoving dust density parameter \u3a9dust and find agreement with data for z < 0.5 in the framework of DE and alternative scenarios. Finally, the evolution of the average cosmic metallicity is presented and it shows a quite fast increase in each scenario, reaching the solar value at the present time, although most of the heavy elements are incorporated into solid grains, and therefore not observable in the gas phase
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