1,721,013 research outputs found
An inhomogeneous model for the Galactic halo: a possible explanation for the spread observed in s- and r-process elements
No full textAims. We propose an explanation for the considerable scatter
of the abundances of neutron capture elements observed in low-metallicity stars
in the solar vicinity, compared to the small star-to-star scatter
observed for the α-elements.
Methods. We have developed a stochastic
chemical evolution model in which the main assumption is a random formation
of new stars subject to the condition that the cumulative mass distribution
follows a given initial mass function.
Results. With our model, we are able to reproduce the different spreads of neutron
capture elements and α-elements in low-metallicity stars.
Conclusions. The reason for different observed spreads in neutron
capture elements and α-elements resides in the random birth of stars, coupled
with different stellar mass ranges, from which α-elements and neutron capture
elements originate. In particular, the site of production of α-elements is the
whole range of massive stars, from 10 to 80
whereas the mass range of production for neutron capture
elements lies between 12 and 30
Manganese evolution in Omega Centauri: a clue to the cluster formation mechanisms?
No full textWe model the evolution of manganese relative to iron in the progenitor system of the globular cluster Omega Centauri by means of a self-consistent chemical evolution model. We use stellar yields that already reproduce the measurements of [Mn/Fe] versus [Fe/H] in Galactic field disc and halo stars, in Galactic bulge stars and in the Sagittarius dwarf spheroidal galaxy. We compare our model predictions to the Mn abundances measured in a sample of 10 red giant members and six subgiant members of ω Cen. The low values of [Mn/Fe] observed in a few, metal-rich stars of the sample cannot be explained in the framework of our standard, homogeneous chemical evolution model. Introducing cooling flows that selectively bring to the cluster core only the ejecta from specific categories of stars does not help to heal the disagreement with the observations. The capture of field stars does not offer a viable explanation either. The observed spread in the data and the lowest [Mn/Fe] values could, in principle, be understood if the system experienced inhomogeneous chemical evolution. Such an eventuality is qualitatively discussed in this paper. However, more measurements of Mn in ω Cen stars are needed to settle the issue of Mn evolution in this cluster
The neutron-capture and alpha-elements abundance ratios scatter in old stellar populations. Cosmological simulations of the stellar halo
Full text linkWe investigate the origin of the abundance ratios and scatter of the
neutron-capture elements Sr, Ba and Eu in the stellar halo of a Milky Way-mass
galaxy formed in a hydrodynamical cosmological simulation, and compare them
with those of -elements. For this, we implement a novel treatment for
chemical enrichment of Type II supernovae which considers the effects of the
rotation of massive stars on the chemical yields and differential enrichment
according to the life-times of progenitor stars. We find that differential
enrichment has a significant impact on the early enrichment of the interstellar
medium which is translated into broader element ratio distributions,
particularly in the case of the oldest, most metal-poor stars. We find that the
[element/Fe] ratios of the elements O, Mg and Si have systematically
lower scatter compared to the neutron-capture elements ratios Sr, Ba and Eu at
[Fe/H], which is dex for the former and between
and dex for the latter. The different scatter levels found for the
neutron-capture and -elements is consistent with observations of old
stars in the Milky Way. Our model also predicts a high scatter for the [Sr/Ba]
ratio, which results from the treatment of the fast-rotating stars and the
dependence of the chemical yields on the metallicity, mass and rotational
velocities. Such chemical patterns appear naturally if the different ejection
times associated to stars of different mass are properly described, without the
need to invoke for additional mixing mechanisms or a distinct treatment of the
and neutron-capture elements.Comment: MNRAS accepte
Silicon depletion in damped Ly α systems. The S/Zn method
No full textilicates are an important component of interstellar dust that has been poorly investigated in high redshift galaxies. As a preliminary step to studying silicates at high redshift, we survey silicon depletions in damped Ly α (DLA) systems. Silicon depletion is mild in the Galactic interstellar medium (ISM) and is expected to be weaker in most DLA systems, so we introduce a method for improving the accuracy of DLA depletion measurements. We compare abundance ratios measured in the gas with calculations of total abundance ratios of gas and dust predicted by models of galactic chemical evolution tailored for DLA systems. To tune the model parameters, we use the dust-free observational diagram S/Zn versus Zn/H, and we also compare the look back time estimated from the absorption redshift with the evolutionary time predicted by the model. By applying our method to a large set of DLA column densities, we succeeded in measuring the depletion of silicon in 74 systems. For comparison, we also measure iron and magnesium depletions (105 and 10 systems, respectively) with the same method. The mean depletion of silicon that we derive, ⟨ δSi ⟩ ≃ -0.27 ± 0.16 dex, is surprisingly close to that of iron, ⟨ δFe ⟩ ≃ -0.42 ± 0.28 dex, despite iron being much more depleted than silicon in the Galactic ISM. Silicon depletion in DLA systems does not correlate with metallicity, at variance with iron depletion, for which we confirm a rise with [Fe/H] found in previous work. Magnesium depletion seems to behave more in accordance with silicon than with iron. The different behaviors of the silicon and iron depletions suggests a complex history of dust production at the early stages of galactic chemical evolution
The fall of a giant. Chemical evolution of Enceladus, alias the Gaia sausage
Full text linkWe present the first chemical evolution model for Enceladus, alias the Gaia Sausage, to investigate the star formation history of one of the most massive satellites accreted by the Milky Way during a major merger event. Our best chemical evolution model for Enceladus nicely fits the observed stellar [α/Fe]–[Fe/H] chemical abundance trends, and reproduces the observed stellar metallicity distribution function, by assuming low star formation efficiency, fast infall time-scale, and mild outflow intensity. We predict a median age for Enceladus stars 12.33+0.92−1.36 Gyr, and – at the time of the merger with our Galaxy (≈10 Gyr ago from Helmi et al.) – we predict for Enceladus a total stellar mass M⋆ ≈ 5 × 109 M⊙. By looking at the predictions of our best model, we discuss that merger events between the Galaxy and systems like Enceladus may have inhibited the gas accretion on to the Galaxy disc at high redshifts, heating up the gas in the halo. This scenario could explain the extended period of quenching in the star formation activity of our Galaxy about 10 Gyr ago, which is predicted by Milky Way chemical evolution models, in order to reproduce the observed bimodality in [α/Fe]–[Fe/H] between thick- and thin-disc stars
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Origin of neutron capture elements with the Gaia-ESO survey: the evolution of s- and r-process elements across the Milky Way
Full text linkWe study the abundance patterns and the radial gradients of s-process
elements (Y, Zr, Ba, La and Ce), r-process elements (Eu) and mixed-process
elements (Mo, Nd and Pr) in the Galactic thin disc by means of a detailed
two-infall chemical evolution model for the Milky Way with state-of-the-art
nucleosynthesis prescriptions. We consider r-process nucleosynthesis from
merging neutron stars (MNS), magneto-rotational supernovae (MR-SNe) and
s-process synthesis from low- and intermediate- mass stars (LIMS) and rotating
massive stars. The predictions of our model are compared with data from the
sixth data release of the Gaia-ESO survey, from which we consider 62 open
clusters with age > 0.1 Gyr and 1300 Milky Way disc field stars. We conclude
that: i) the [Eu/Fe] vs. [Fe/H] is reproduced by both a prompt and a delayed
source, but the quick source completely dominates the Eu production; ii)
rotation in massive stars contribute substantially to the s-process elements of
the first peak, but MNS and MR-SNe are necessary in order to reproduce the
observations; iii) due to the adopted yields, our model overpredicts Pr and
underpredicts Nd, while the [Mo/Fe] vs. [Fe/H] is nicely reproduced. For the
radial gradients, we conclude that: i) our predicted slope of the [Fe/H]
gradient is in agreement with the one observed in open clusters by Gaia-ESO and
other high-resolution spectroscopic surveys. ii) The predicted slope of the
[Eu/H] radial gradient is steeper than the observed one, independently on how
quick the production of Eu is. We discuss the possible causes of this
discrepancy in terms of both different Galaxy formation scenarios and stellar
radial migration effects. iii) For all the elements belonging to the second
s-process peak (Ba, La, Ce) as well as for Pr, we predict a plateau at low
Galactocentric distances, which is probably due to the enhanced enrichment from
LIMS in the inner regions.Comment: 17 pages, 11 figures. Published on MNRA
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