2,536 research outputs found
Effects of thermohaline instability and rotation-induced mixing on the evolution of light elements in the Galaxy : D, 3He and 4He
Recent studies of low- and intermediate-mass stars show that the evolution of
the chemical elements in these stars is very different from that proposed by
standard stellar models. Rotation-induced mixing modifies the internal chemical
structure of main sequence stars, although its signatures are revealed only
later in the evolution when the first dredge-up occurs. Thermohaline mixing is
likely the dominating process that governs the photospheric composition of
low-mass red giant branch stars and has been shown to drastically reduce the
net 3He production in these stars. The predictions of these new stellar models
need to be tested against galaxy evolution. In particular, the resulting
evolution of the light elements D, 3He and 4He should be compared with their
primordial values inferred from the Wilkinson Microwave Anisotropy Probe data
and with the abundances derived from observations of different Galactic
regions. We study the effects of thermohaline mixing and rotation-induced
mixing on the evolution of the light elements in the Milky Way. We compute
Galactic evolutionary models including new yields from stellar models computed
with thermohaline instability and rotation-induced mixing. We discuss the
effects of these important physical processes acting in stars on the evolution
of the light elements D, 3He, and 4He in the Galaxy. Galactic chemical
evolution models computed with stellar yields including thermohaline mixing and
rotation fit better observations of 3He and 4He in the Galaxy than models
computed with standard stellar yields. The inclusion of thermohaline mixing in
stellar models provides a solution to the long-standing "3He problem" on a
Galactic scale. Stellar models including rotation-induced mixing and
thermohaline instability reproduce also the observations of D and 4He.Comment: 12 pages, 9 figures, accepted for publication in A&
Evolution of N/O ratios in galaxies from cosmological hydrodynamical simulations
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We study the redshift evolution of the gas-phase O/H and N/O abundances, both (i) for individual interstellar medium (ISM) regions within single spatially resolved galaxies and (ii) when dealing with average abundances in the whole ISM of many unresolved galaxies. We make use of a cosmological hydrodynamical simulation including detailed chemical enrichment, which properly takes into account the variety of different stellar nucleosynthetic sources of O and N in galaxies. We identify 33 galaxies in the simulation, lying within dark matter haloes with virial mass in the range 10 11 ≤ M DM ≤ 10 13 M ⊙ and reconstruct how they evolved with redshift. For the local and global measurements, the observed increasing trend of N/O at high O/H can be explained, respectively, (i) as the consequence of metallicity gradients that have settled in the galaxy ISM, where the innermost galactic regions have the highest O/H abundances and the highest N/O ratios, and (ii) as the consequence of an underlying average mass-metallicity relation that galaxies obey as they evolve across cosmic epochs, where - at any redshift - less massive galaxies have lower average O/H and N/O ratios than the more massive ones. We do not find a strong dependence on the environment. For both local and global relations, the predicted N/O-O/H relation is due to the mostly secondary origin of N in stars. We also predict that the O/H and N/O gradients in the galaxy ISM gradually flatten as functions of redshift, with the average N/O ratios being strictly coupled with the galaxy star formation history. Because N production strongly depends on O abundances, we obtain a universal relation for the N/O-O/H abundance diagram whether we consider average abundances of many unresolved galaxies put together or many abundance measurements within a single spatially resolved galaxy.Peer reviewedFinal Accepted Versio
A spectroscopic binary in the Hercules dwarf spheroidal galaxy
We present the radial velocity curve of a single-lined spectroscopic binary
in the faint Hercules dwarf spheroidal (dSph) galaxy, based on 34 individual
spectra covering more than two years of observations. This is the first time
that orbital elements could be derived for a binary in a dSph. The system
consists of a metal-poor red giant and a low-mass companion, possibly a white
dwarf, with a 135-days period in a moderately eccentric () orbit. Its
period and eccentricity are fully consistent with metal-poor binaries in the
Galactic halo, while the projected semimajor axis is small, at sin =
38 R. In fact, a very close orbit could inhibit the production of
heavier elements through -process nucleosynthesis, leading to the very low
abundances of neutron-capture elements that are found in this star. We discuss
the further implications for the chemical enrichment history of the Hercules
dSph, but find no compelling binary scenario that could reasonably explain the
full, peculiar abundance pattern of the Hercules dSph galaxy.Comment: 7 pages, 3 figures, accepted for publication in the Astrophysical
Journa
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
Kinematic Evidence for Superbubbles in I Zw 18: Constraints on the Star Formation History and Chemical Evolution
We have combined measurements of the kinematics, morphology, and oxygen
abundance of the ionized gas in \IZw18, one of the most metal-poor galaxies
known, to examine the star formation history and chemical mixing processes.Comment: 31 pages including 6 figures. Accepted for publication in the
Astrophysical Journa
Galactic and Cosmic Type Ia SN rates: is it possible to impose constraints on SNIa progenitors?
We compute the Type Ia supernova rates in typical elliptical galaxies by
varying the progenitor models for Type Ia supernovae. To do that a formalism
which takes into account the delay distribution function (DTD) of the explosion
times and a given star formation history is adopted. Then the chemical
evolution for ellipticals with baryonic initial masses , and
is computed, and the mass of Fe produced by each galaxy is
precisely estimated. We also compute the expected Fe mass ejected by
ellipticals in typical galaxy clusters (e.g. Coma and Virgo), under different
assumptions about Type Ia SN progenitors. As a last step, we compute the cosmic
Type Ia SN rate in an unitary volume of the Universe by adopting several cosmic
star formation rates and compare it with the available and recent observational
data. Unfortunately, no firm conclusions can be derived only from the cosmic
SNIa rate, neither on SNIa progenitors nor on the cosmic star formation rate.
Finally, by analysing all our results together, and by taking into account
previous chemical evolution results, we try to constrain the best Type Ia
progenitor model. We conclude that the best progenitor models for Type Ia SNe
are still the single degenerate model, the double degenerate wide model, and
the empirical bimodal model. All these models require the existence of prompt
Type Ia supernovae, exploding in the first 100 Myr since the beginning of star
formation, although their fraction should not exceed 15-20% in order to fit
chemical abundances in galaxies.Comment: 17 pages, 11 figures, Submitted to MNRA
Formation and evolution of late-type dwarf galaxies. I. NGC 1705 and NGC 1569
(Abridged.) We present one-zone chemical evolution models for two dwarf
starburst galaxies, NGC 1705 and NGC 1569. Using information about the past
star formation history and initial mass function of the systems previously
obtained from Hubble Space Telescope colour-magnitude diagrams, we identify
possible scenarios of chemical enrichment and development of galactic winds. In
order not to overestimate the current metallicity of the interstellar gas
inferred from H II region spectroscopy, we suggest that the winds efficiently
remove from the galaxies the metal-rich ejecta of dying stars. Conversely,
requiring the final mass of neutral gas to match the value inferred from 21-cm
observations implies a relatively low efficiency of interstellar medium
entrainment in the outflow, thus confirming previous findings that the winds
driving the evolution of typical starbursts are differential. These conclusions
could be different only if the galaxies accrete huge fractions of unprocessed
gas at late times. By assuming standard stellar yields we obtain a good fit to
the observed nitrogen to oxygen ratio of NGC 1569, while the mean N/O ratio in
NGC 1705 is overestimated by the models. Reducing the extent of hot bottom
burning in low-metallicity intermediate-mass stars does not suffice to solve
the problem. Localized self-pollution from stars more massive than 60 MSun in
NGC 1705 and/or funneling of larger fractions of nitrogen through its winds are
then left to explain the discrepancy between model predictions and
observations. Inspection of the log(N/O) vs. log(O/H)+12 diagram for a sample
of dwarf irregular and blue compact dwarf galaxies in the literature favours
the latter hypothesis.Comment: 20 pages, 10 figures, accepted for publication on MNRA
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
The formation of the [alpha/Fe] radial gradients in the stars of elliptical galaxies
The scope of this paper is two-fold: i) to test and improve our previous
models of an outside-in formation for the majority of ellipticals in the
context of the SN-driven wind scenario, by means of a careful study of gas
inflows/outflows; ii) to explain the observed slopes, either positive or
negative, in the radial gradient of the mean stellar [alpha/Fe], and their
apparent lack of any correlation with all the other observables. In order to
pursue these goals we present a new class of hydrodynamical simulations for the
formation of single elliptical galaxies in which we implement detailed
prescriptions for the chemical evolution of H, He, O and Fe. We find that all
the models which predict chemical properties (such as the central mass-weighted
abundance ratios, the colours as well as the [] gradient) within the
observed ranges for a typical elliptical, also exhibit a variety of gradients
in the [] ratio, in agreement with the observations (namely positive,
null or negative). All these models undergo an outside-in formation, in the
sense that star formation stops earlier in the outermost than in the innermost
regions, owing to the onset of a galactic wind. The typical [] gradients
predicted by our models have a slope of -0.3 dex per decade variation in
radius, consistent with the mean values of several observational samples. We
can safely conclude that the history of star formation is fundamental for the
creation of abundance gradients in ellipticals but that radial flows with
different velocity in conjunction with the duration and efficiency of star
formation in different galactic regions are responsible for the gradients in
the [] ratios.Comment: A&A accepted, replaced with final version after the peer-review
proces
- …