496 research outputs found
New chemical evolution analytical solutions including environment effects
In the last years, more and more interest has been devoted to analytical
solutions, including inflow and outflow, to study the metallicity enrichment in
galaxies. In this framework, we assume a star formation rate which follows a
linear Schmidt law, and we present new analytical solutions for the evolution
of the metallicity (Z) in galaxies. In particular, we take into account
environmental effects including primordial and enriched gas infall, outflow,
different star formation efficiencies, and galactic fountains. The enriched
infall is included to take into account galaxy-galaxy interactions. Our main
results can be summarized as: i) when a linear Schmidt law of star formation is
assumed, the resulting time evolution of the metallicity Z is the same either
for a closed-box model or for an outflow model. ii) The mass-metallicity
relation for galaxies which suffer a chemically enriched infall, originating
from another evolved galaxy with no pre-enriched gas, is shifted down in
parallel at lower Z values, if compared the closed box model. iii) When a
galaxy suffers at the same time a primordial infall and a chemically enriched
one, the primordial infall always dominates the chemical evolution. iv) We
present new solutions for the metallicity evolution in a galaxy which suffers
galactic fountains and an enriched infall from another galaxy at the same time.
The analytical solutions presented here can be very important to study the
metallicity (oxygen), which is measured in high-redshift objects. These
solutions can be very useful: a) in the context of cosmological semi-analytical
models for galaxy formation and evolution, and b) for the study of compact
groups of galaxies.Comment: Accepted for publication in MNRA
Chemical Evolution of M31
We review chemical evolution models developed for M31 as well as the
abundance determinations available for this galaxy. Then we present a recent
chemical evolution model for M31 including radial gas flows and galactic
fountains along the disk, as well as a model for the bulge. Our models are
predicting the evolution of the abundances of several chemical species such as
H, He, C, N, O, Ne, Mg, Si, S, Ca and Fe. From comparison between model
predictions and observations we can derive some constraints on the evolution of
the disk and the bulge of M31. We reach the conclusions that Andromeda must
have evolved faster than the Milky Way and inside-out, and that its bulge
formed much faster than the disk on a timescale 0.5 Gyr. Finally, we
present a study where we apply the model developed for the disk of M31 in order
to study the probability of finding galactic habitable zones in this galaxy.Comment: To be published in:"Lessons from the Local Group: A Conference in
Honour of David Block and Bruce Elmegreen" Editors: Prof. Dr. Kenneth
Freeman, Dr. Bruce Elmegreen, Prof. Dr. David Block, Matthew Woolway,
Springe
The Galactic habitable zone around M and FGK stars with chemical evolution models with dust
The Galactic habitable zone is defined as the region with highly enough
metallicity to form planetary systems in which Earth-like planets could be born
and might be capable of sustaining life surviving to the destructive effects of
nearby supernova explosion events. Galactic chemical evolution models can be
useful tools for studying the galactic habitable zones in different systems.
Our aim here is to find the Galactic habitable zone using chemical evolution
models for the Milky Way disc, adopting the most recent prescriptions for the
evolution of dust and for the probability of finding planetary systems around M
and FGK stars. Moreover, for the first time, we will express those
probabilities in terms of the dust-to-gas ratio of the ISM in the solar
neighborhood as computed by detailed chemical evolution models. At a fixed
Galactic time and Galactocentric distance we determine the number of M and FGK
stars having Earths (but no gas giant planets) which survived supernova
explosions, using the formalism of our Paper I. The probabilities of finding
terrestrial planets but not gas giant planets around M stars deviate
substantially from the ones around FGK stars for supersolar values of [Fe/H].
For both FGK and M stars the maximum number of stars hosting habitable planets
is at 8 kpc from the Galactic Centre, if destructive effects by supernova
explosions are taken into account. At the present time the total number of M
stars with habitable planets are 10 times the number of FGK stars.
Moreover, we provide a sixth order polynomial fit (and a linear one but more
approximated) for the relation found with chemical evolution models in the
solar neighborhood between the [Fe/H] abundances and the dust-to-gas ratio.Comment: Accepted for publication in A&A, 10 pages 6 figure
The galactic habitable zone of the Milky Way and M31 from chemical evolution models with gas radial flows
The galactic habitable zone is defined as the region with sufficient
abundance of heavy elements to form planetary systems in which Earth-like
planets could be born and might be capable of sustaining life, after surviving
to close supernova explosion events. Galactic chemical evolution models can be
useful for studying the galactic habitable zones in different systems. We apply
detailed chemical evolution models including radial gas flows to study the
galactic habitable zones in our Galaxy and M31. We compare the results to the
relative galactic habitable zones found with "classical" (independent ring)
models, where no gas inflows were included. For both the Milky Way and
Andromeda, the main effect of the gas radial inflows is to enhance the number
of stars hosting a habitable planet with respect to the "classical" model
results, in the region of maximum probability for this occurrence, relative to
the classical model results. These results are obtained by taking into account
the supernova destruction processes. In particular, we find that in the Milky
Way the maximum number of stars hosting habitable planets is at 8 kpc from the
Galactic center, and the model with radial flows predicts a number which is 38%
larger than what predicted by the classical model. For Andromeda we find that
the maximum number of stars with habitable planets is at 16 kpc from the center
and that in the case of radial flows this number is larger by 10 % relative to
the stars predicted by the classical model.Comment: Accepted by MNRA
Abundance gradients in spiral disks: is the gradient inversion at high redshift real?
We compute the abundance gradients along the disk of the Milky Way by means
of the two-infall model: in particular, the gradients of oxygen and iron and
their temporal evolution. First, we explore the effects of several physical
processes which influence the formation and evolution of abundance gradients.
They are: i) the inside-out formation of the disk, ii) a threshold in the gas
density for star formation, iii) a variable star formation efficiency along the
disk, iv) radial flows and their speed, and v) different total surface mass
density (gas plus stars) distributions for the halo. We are able to reproduce
at best the present day gradients of oxygen and iron if we assume an inside-out
formation, no threshold gas density, a constant efficiency of star formation
along the disk and radial gas flows. It is particularly important the choice of
the velocity pattern for radial flows and the combination of this velocity
pattern with the surface mass density distribution in the halo. Having selected
the best model, we then explore the evolution of abundance gradients in time
and find that the gradients in general steepen in time and that at redshift z~3
there is a gradient inversion in the inner regions of the disk, in the sense
that at early epochs the oxygen abundance decreases toward the Galactic center.
This effect, which has been observed, is naturally produced by our models if an
inside-out formation of the disk and and a constant star formation efficiency
are assumed. The inversion is due to the fact that in the inside-out formation
a strong infall of primordial gas, contrasting chemical enrichment, is present
in the innermost disk regions at early times. The gradient inversion remains
also in the presence of radial flows, either with constant or variable speed in
time, and this is a new result.Comment: 15 pages, 19 figures, accepted for publication in MNRA
The effect of stellar migration on Galactic chemical evolution: a heuristic approach
In the last years, stellar migration in galactic discs has been the subject
of several investigations. However, its impact on the chemical evolution of the
Milky Way still needs to be fully quantified. In this paper, we aim at imposing
some constraints on the significance of this phenomenon by considering its
influence on the chemical evolution of the Milky Way thin disc. We do not
investigate the physical mechanisms underlying the migration of stars. Rather,
we introduce a simple, heuristic treatment of stellar migration in a detailed
chemical evolution model for the thin disc of the Milky Way, which already
includes radial gas flows and reproduces several observational constraints for
the solar vicinity and the whole Galactic disc. When stellar migration is
implemented according to the results of chemo-dynamical simulations by Minchev
et. al. (2013) and finite stellar velocities of 1 km s are taken into
account, the high-metallicity tail of the metallicity distribution function of
long-lived thin-disc stars is well reproduced. By exploring the velocity space,
we find that the migrating stars must travel with velocities in the range 0.5
-2 km s to properly reproduce the high-metallicity tail of the
metallicity distribution. We confirm previous findings by other authors that
the observed spread in the age-metallicity relation of solar neighbourhood
stars can be explained by the presence of stars which originated at different
Galactocentric distances, and we conclude that the chemical properties of stars
currently observed in the solar vicinity do suggest that stellar migration is
present to some extent.Comment: Accepted for publication by Ap
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&
Ergodicity versus non-ergodicity for Probabilistic Cellular Automata on rooted trees
In this article we study a class of shift-invariant and positive rate
probabilistic cellular automata (PCA) on rooted d-regular trees .
In a first result we extend the results of [10] on trees, namely we prove
that to every stationary measure of the PCA we can associate a space-time
Gibbs measure on
. Under certain assumptions on the dynamics
the converse is also true.
A second result concerns proving sufficient conditions for ergodicity and
non-ergodicity of our PCA on d-ary trees for and
characterizing the invariant product Bernoulli measures.Comment: 17 page
Homogeneous nucleation for Glauber and Kawasaki dynamics in large volumes at low temperatures
In this paper we study metastability in large volumes at low temperatures. We
consider both Ising spins subject to Glauber spin-flip dynamics and lattice gas
particles subject to Kawasaki hopping dynamics. Let \b denote the inverse
temperature and let \L_\b \subset \Z^2 be a square box with periodic boundary
conditions such that \lim_{\b\to\infty}|\L_\b|=\infty. We run the dynamics on
\L_\b starting from a random initial configuration where all the droplets (=
clusters of plus-spins, respectively, clusters of particles)are small. For
large \b, and for interaction parameters that correspond to the metastable
regime, we investigate how the transition from the metastable state (with only
small droplets) to the stable state (with one or more large droplets) takes
place under the dynamics. This transition is triggered by the appearance of a
single \emph{critical droplet} somewhere in \L_\b. Using potential-theoretic
methods, we compute the \emph{average nucleation time} (= the first time a
critical droplet appears and starts growing) up to a multiplicative factor that
tends to one as \b\to\infty. It turns out that this time grows as
Ke^{\Gamma\b}/|\L_\b| for Glauber dynamics and K\b e^{\Gamma\b}/|\L_\b| for
Kawasaki dynamics, where is the local canonical, respectively,
grand-canonical energy to create a critical droplet and is a constant
reflecting the geometry of the critical droplet, provided these times tend to
infinity (which puts a growth restriction on |\L_\b|). The fact that the
average nucleation time is inversely proportional to |\L_\b| is referred to
as \emph{homogeneous nucleation}, because it says that the critical droplet for
the transition appears essentially independently in small boxes that partition
\L_\b.Comment: 45 pages, 11 figure
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