G. Cescutti

Chromium: NLTE abundances in metal-poor stars and nucleosynthesis in the Galaxy

Aims. We investigate statistical equilibrium of Cr in the atmospheres of late-type stars to ascertain whether the systematic abundance discrepancy between Cr I and Cr II lines, as often found in previous work, is due to deviations from local thermodynamic equilibrium (LTE). Furthermore, we attempt to interpret the Non-LTE (NLTE) trend of [Cr/Fe] with [Fe/H] using chemical evolution models for the solar neighborhood. Methods. NLTE calculations are performed for the model of the Cr atom, comprising 340 levels and 6806 transitions in total. We use the quantum-mechanical photoionization cross-sections of Nahar (2009) and investigate the sensitivity of the model to uncertain crosssections for H I collisions. NLTE line formation is performed for the MAFAGS-ODF model atmospheres of the Sun and 10 metal-poor stars with −3.2 < [Fe/H] < −0.5, and Cr abundances are derived by comparing the synthetic and observed flux spectra. Results. We achieve good ionization equilibrium of Cr for models with different stellar parameters, if inelastic collisions with H I atoms are neglected. The solar NLTE abundance based on Cr I lines is 5.74 dex with σ = 0.05 dex, which is ∼0.1 dex higher than the LTE abundance. For the metal-poor stars, the NLTE abundance corrections to Cr I lines range from +0. 3t o+0.5 dex. The resulting [Cr/Fe] ratio is roughly solar for the range of metallicities analyzed here, which is consistent with current views on the production of these iron peak elements in supernovae. Conclusions. The tendency of Cr to become deficient with respect to Fe in metal-poor stars is an artifact caused by the neglect of NLTE effects in the line formation of Cr i, and has no relation to any peculiar physical conditions in the Galactic ISM or deficiencies of nucleosynthesis theory.

An inhomogeneous model for the Galactic halo: a possible explanation for the spread observed in s- and r-process elements

Aims. We propose an explanation for the considerable scatter of the abundances of neutron capture elements observed in lowmetallicity 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

The evolution of carbon and oxygen in the bulge and disk of the Milky Way

Context. The evolution of C and O abundances in the Milky Way can impose strong constraints on stellar nucleosynthesis and help in understanding the formation and evolution of our Galaxy. Aims. The aim of this paper is to review the measured C and O abundances in the disk and bulge of the Galaxy and compare the results to predictions of Galactic chemical evolution models. Methods. We adopt two successful chemical evolution models for the bulge and the disk, respectively. They assume the same nucleosynthesis prescriptions but different histories of star formation. Results. The data show a clear distinction between the trend of [C/O] in the thick and thin Galactic disks, while the thick disk and bulge trends are indistinguishable with a large (>0.5 dex) increase in the [C/O] ratio in the range from -0.1 to +0.4 dex for [O/H]. In our models we consider yields from massive stars with and without the inclusion of metallicity-dependent stellar winds. The observed increase in the [C/O] with metallicity in the bulge and thick disk lies between the predictions utilizing mass-loss rates of Maeder and Meynet & Maeder. A model without metallicity-dependent yields completely fails to match the observations. Thus, the relative increase in carbon abundance at high metallicity appears to come from metallicity-dependent stellar winds in massive stars. These results also explain the steep decline of the [O/Fe] ratio with [Fe/H] in the Galactic bulge, while the [Mg/Fe] ratio is enhanced at all [Fe/H]. Conclusions. We conclude that data and models are consistent with a rapid bulge and thick disk formation timescales, and with metallicity-dependent yields for C and O. The observed too high [C/O] ratios at low metallicity in the bulge may stem from an unaccounted source of carbon: very fast rotating metal poor stars, or metal-poor binary systems whose envelopes were stripped by Roche lobe overflow.

The chemical evolution of manganese in different stellar systems

Aims. We model the chemical evolution of manganese relative to iron in three different stellar systems: the Solar neighbourhood, the Galactic bulge, and the Sagittarius dwarf spheroidal galaxy, and we compare our results with recent and homogeneous observational data sets. Methods. We adopt three chemical evolution models able to reproduce the main properties of the Solar vicinity, the Galactic bulge, and the Sagittarius dwarf spheroidal. We then compare different stellar yields in order to identify the most appropriate set to match the observational data in these systems. Results. We compute the evolution of manganese in the three systems and find that to reproduce simultaneously the measurements of [Mn/Fe] versus [Fe/H] in the Galactic bulge, the Solar neighbourhood and Sagittarius, the type Ia supernova (SN) Mn yield must be metallicity-dependent. Conclusions. We conclude that modelling different histories of star formation in the three systems are insufficient to reproduce the different behaviour of the [Mn/Fe] ratio, unlike the situation for [α/Fe]; rather, it is necessary to invoke metallicity-dependent type Ia SN Mn yields, as originally suggested by McWilliam, Rich & Smecker-Hane.

LTE or non-LTE, that is the question - The NLTE chemical evolution of strontium in extremely metal-poor stars

Strontium has proven itself to be one of the most important neutron-capture elements in the study of metal-poor stars. Thanks to the strong absorption lines of Sr, they can be detected even in the most metal-poor stars and also in low-resolution spectra. However, we still cannot explain the large star-to-star abundance scatter we derive for metal-poor stars. Here we contrast Galactic chemical evolution (GCE) with improved abundances for SrI+II including updated atomic data, to evaluate possible explanations for the large star-to-star scatter at low metallicities. We derive abundances under both local thermodynamic equilibrium (LTE) and non-LTE (NLTE) for stars spanning a large interval of stellar parameters. Gravities and metallicities are also determined in NLTE. We confirm that the ionisation equilibrium between SrI and SrII is satisfied under NLTE but not LTE, where the difference between SrI and SrII is on average ~0.3dex. We show that the NLTE corrections are of increasing importance as the metallicity decreases. For the stars with [Fe/H]>-3 the SrI NLTE correction is ~0.35/0.55dex in dwarfs/giants, while the Sr II NLTE correction is +/-0.05dex. On the basis of the large NLTE corrections, SrI should not be applied as a chemical tracer under LTE, while it is a good tracer under NLTE. SrII is a good tracer under both LTE and NLTE (down to [Fe/H]\sim -3), and LTE is a safe assumption for this majority species. However, the Sr abundance from SrII lines is dependent on an accurate surface gravity determination, which can be obtained from NLTE spectroscopy of Fe lines or from parallax measurements. We could not explain the star-to-star scatter (which remains under both LTE and NLTE) by the use of the GCE model, since the Sr yields to date are too uncertain to draw firm conclusions. At least two production sites seem necessary in order to account for this large scatter (abridged).

Detailed chemical evolution of Carina and Sagittarius dwarf spheroidal galaxies

Aims. In order to verify the effects of the most recent data on the evolution of Carina and Sagittarius Dwarf Spheroidal Galaxies (dSph) and to set tight constraints on the main parameters of chemical evolution models, we study in detail the chemical evolution of these galaxies through comparisons between the new data and the predictions of a model, already tested to reproduce the main observational constraints in dSphs. Methods. Several abundance ratios, such as [�/Fe], [Ba/Fe] and [Eu/Fe], and the metallicity distribution of stars are compared to the predictions of our models adopting the observationally derived star formation histories in these galaxies. Results. These new comparisons confirm our previously suggested scen ario for the evolution of these galaxies, and allow us to bett er fix the star formation and wind parameters. In particular, fo r Carina our predictions indicates that the best effi ciency of star formation is � = 0.15 Gyr −1 , that the best wind effi ciency parameter is wi = 5 (the wind rate is five times stronger than the star formation rate), and that the star formation history, which produces the best fit to the observed metallicity distribution of stars is characteriz ed by several episodes of activity. In the case of Sagittarius our results suggest tha t � = 3 Gyr −1 and wi = 9, again in agreement with our previous work. Finally, we show new predictions for [N/Fe] and [C/Fe] ratios for the two galaxies suggesting a scenario for Sagittarius very similar to the one of the solar vicinity in the Milky Way, except for a slight decrease of [N/Fe] ratio at high metallicities due to the galactic wind. For Carina we predict a larger [N/Fe] ratio at low metallicities, reflecting the lower star for mation effi ciency of this galaxy relative to Sagittarius and the Milky Way.

The role of neutron star mergers in the chemical evolution of the Galactic halo

Aims. We explore the problem of the site of production of Eu. We use also the information present in the observed spread in the Eu abundances in the early Galaxy, not only its average trend. Moreover, we extend to other heavy elements (Ba, Sr, Rb, Zr) our investigations to provide additional constraints to our results. Methods. We adopt a stochastic chemical evolution model taking into account inhomogeneous mixing. The adopted yields of Eu from neutron star mergers (NSM) and from core-collapse supernovae (SNII) are those that are able to explain the average [Eu/Fe]-[Fe/H] trend observed for solar neighborhood stars, in the framework of a well-tested homogeneous model for the chemical evolution of the MilkyWay. Rb, Sr, Zr, and Ba are produced by both the s- and r-process. The s-process contribution by spinstars is the same as in our previous papers. Results. NSM that merge in less than 10 Myr or NSM combined with a source of r-process generated by massive stars can explain the spread of [Eu/Fe] in the Galactic halo. The combination of r-process production by NSM and s-process production by spinstars is able to reproduce the available observational data for Sr, Zr and Ba. We also show the first predictions for Rb in the Galactic halo. Conclusions. We confirm previous results that either NSM with very short time scale or both NSM and at least a fraction of SNII should have contributed to the synthesis of Eu in the Galaxy. The r-process production by NSM - complemented by an s-process production by spinstars - provide results compatible with our previous findings based on other r-process sites. We critically discuss the weak and strong points of both NSM and SNII scenarios for producing Eu and eventually suggest that the best solution is probably a mixed one in which both sources produce Eu. In fact, this scenario better reproduces the scatter observed in all the studied elements. [abridged]

A comparison of the s- and r-process element evolution in local dwarf spheroidal galaxies and in the Milky Way

Aims. We study the nucleosynthesis of several neutron capture elements (barium, europium, lanthanum, and yttrium) in local group dwarf spheroidal (dSph) galaxies and in the Milky Way by comparing the predictions of detailed chemical evolution models with the observed data. We analyse the differences in the abundance patterns of these two types of galaxies in order to understand their formation and evolution. Methods. We compare the evolution of [Ba/Fe], [Eu/Fe], [La/Fe], [Y/Fe], [Ba/Y], [Ba/Eu], [Y/Eu], and [La/Eu] observed in dSph galaxies and in our Galaxy with predictions of detailed chemical evolution models. The models for all dSph galaxies and for the Milky Way are able to reproduce several observational features of these galaxies, such as a series of abundance ratios and the stellar metallicities distributions. The Milky Way model adopts the two-infall scenario, whereas the most important features of the models for the dSph galaxies are the low star-formation rate and the occurrence of intense galactic winds. Results. We predict that the [s-r/Fe] ratios in dSphs are generally different than the corresponding ratios in the Milky Way, at the same [Fe/H] values. This is interpreted as a consequence of the time-delay model coupled with different star formation histories. In particular, the star-formation is less efficient in dSphs than in our Galaxy and it is influenced by strong galactic winds. Our predictions are in very good agreement with the available observational data. Conclusions. The time-delay model for the galactic chemical enrichment coupled with different histories of star formation in different galaxies allow us to succesfully interpret the observed differences in the abundance ratios of s -a ndr-process elements, as well as of α-elements in dSphs and in the Milky Way. These differences strongly suggest that the main stellar populations of these galaxies could not have had a common origin and, consequently, that the progenitors of local dSphs might not be the same objects as the building blocks of our Galaxy.

Silicon depletion in damped Ly α systems - The S/Zn method

Silicates 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.

Chemical evolution of the Galactic bulge: different stellar populations and possible gradients

Context. The recent although controversial discovery of two main stellar populations in the Galactic bulge, one metal-poor with a spheroid kinematics and the other metal-rich with a bar-like kinematics, suggests a revision of the classical model for bulge formation. Aims. We aim at computing in detail the chemical evolution of the Galactic bulge in order to explain the existence of the two main stellar populations. We also plan to explore the possible existence of spatial abundance gradients inside the bulge. Methods. To do that, we adopt a chemical evolution model that follows the evolution of several chemical species (from H to Ba) and takes into account both infall and outflow of gas. We assume that the metal-poor population formed first and on a short timescale, in agreement with previous models, while the metal-rich population formed later and out of the enriched gas either left from the formation of the previous one or originating from the inner disk. We predict the stellar distribution functions for Fe and Mg, the mean � [Fe/H]� and � [Mg/H]� as well as the [Mg/Fe] vs. [Fe/H] relations in the two stellar populations. Then, we consider the case in which the metal-poor population could be the result of sub-populations formed with different chemical enrichment rates. In particular, the population close to the Galactic center could have evolved very fast, while the more external population could have evolved more slowly, in agreement with the dissipational gravitational collapse scenario. Results. When compared with observations, our results confirm that the old more metal-poor stellar population formed very fast (on a timescale of 0.1–0.3 Gyr) by means of an intense burst of star formation coupled with an initial mass function flatter than in the solar vicinity, but not as flat as suggested in previous works. The metal-rich population, instead, should have formed on a longer timescale (∼3 Gyr). We predict differences in the mean abundances of the two populations: in particular, we find a difference of ∼–0.52 dex for � [Fe/H]� . These differences can be interpreted as a metallicity gradient. We also predict possible gradients for Fe, O, Mg, Si, S, and Ba between sub-populations inside the metal-poor population itself (e.g., –0.145 dex for � [Fe/H]� ). Finally, by means of a chemodynamical model following a dissipational collapse, we predict a gradient inside 500 pc from the Galactic center of −0.26 dex kpc −1 in Fe. Conclusions. We conclude that the chemical evolution of the Galactic bulge, as suggested by its stellar populations, has been quite complex. A stellar population forming by means of a classical gravitational gas collapse is probably mixed with a younger stellar population created perhaps by the bar evolution. The differences among their mean abundances can be interpreted as a gradient. On the basis of both chemical and chemo-dynamical models, we also conclude that it is possible that the metal-poor population itself contains abundance gradients and thus different stellar populations.