1,823 research outputs found
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 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
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 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
Are z>2 Herschel galaxies proto-spheroids?
We present a backward approach for the interpretation of the evolution of the
near-infrared and the far-infrared luminosity functions across the redshift
range 0<z<3. In our method, late-type galaxies are treated by means of a
parametric phenomenological method based on PEP/HerMES data up to z~4, whereas
spheroids are described by means of a physically motivated backward model. The
spectral evolution of spheroids is modelled by means of a single-mass model,
associated to a present-day elliptical with K-band luminosity comparable to the
break of the local early-type luminosity function. The formation of
proto-spheroids is assumed to occurr across the redshift range 1< z < 5. The
key parameter is represented by the redshift z_0.5 at which half
proto-spheroids are already formed. A statistical study indicates for this
parameter values between z_0.5=1.5 and z_0.5=3. We assume as fiducial value
z_0.5~2, and show that this assumption allows us to describe accourately the
redshift distributions and the source counts. By assuming z_0.5 ~ 2 at the
far-IR flux limit of the PEP-COSMOS survey, the PEP-selected sources observed
at z>2 can be explained as progenitors of local spheroids caught during their
formation. We also test the effects of mass downsizing by dividing the
spheroids into three populations of different present-day stellar masses. The
results obtained in this case confirm the validity of our approach, i.e. that
the bulk of proto-spheroids can be modelled by means of a single model which
describes the evolution of galaxies at the break of the present-day early type
K-band LF.Comment: Accepted for publication in ApJ; 26 pages; 13 figure
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
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
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
Stellar Metallicities and SNIa Rates in the Early-type Galaxy NGC5846 from ROSAT and ASCA Observations
In this paper we analyze the diffuse X-ray coronae surrounding the elliptical
galaxy NGC5846, combining measurements from two observatories, ROSAT and ASCA.
We map the gas temperature distribution and find a central cool region within
an approximately isothermal gas halo extending to a radius of about 50 kpc, and
evidence for a temperature decrease at larger radii. With a radially falling
temperature profile, the total mass converges to 9.6+/-1.0 10^12 Msun at ~230
kpc radius. Using the spectroscopic measurements, we also derive radial
distributions for the heavy elements silicon and iron and find that the
abundances of both decrease with galaxy radius. The mass ratio of Si to Fe lies
between the theoretical predictions for element production in SN Ia and SN II,
suggesting an important role for SN Ia, as well as SN II, for gas enrichment in
ellipticals. Using the SN Ia yield of Si, we set an upper limit of 0.012 SNU
for the SN Ia rate at radii >50 kpc, which is independent of possible
uncertainties in the iron L-shell modeling. We compare our observations with
the theoretical predictions for the chemical evolution of ellipticals, taken
from Matteucci & Gibson (1995). We conclude that the metal content in stars, if
explained by the star formation duration, requires a significant decline in the
duration of star formation with galaxy radius, ranging from ~1 Gyr at the
center to ~0.01 Gyr at 100 kpc radius. Alternatively, the decline in
metallicity with galaxy radius may be caused by a similar drop with radius in
the efficiency of star formation. Based on the Si and Fe measurements presented
in this paper, we conclude that the latter scenario is preferred, unless a
dependence of the SN Ia rate on stellar metallicity is invoked. (Abridged).Comment: 11 pages, figures&tables included, emulapj.sty, accepted for Ap
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