Carbon-enhanced metal-poor (CEMP) stars are observed as a substantial fraction of the very metal-poor stars in the Galactic halo. Most CEMP stars are also enriched in s-process elements, and these are often found in binary systems. This suggests that the carbon enrichment is due to mass transfer in the past from an asymptotic giant branch (AGB) star on to a low-mass companion. Models of binary population synthesis are not able to reproduce the observed fraction of CEMP stars without invoking non-standard nucleosynthesis or a substantial change in the initial mass function. This is interpreted as evidence of missing physical ingredients in the models. Recent hydrodynamical simulations show that efficient wind mass transfer is possible in the case of the slow and dense winds typical of AGB stars through a mechanism called wind Roche-lobe overflow (WRLOF), which lies in between the canonical Bondi-Hoyle-Lyttleton (BHL) accretion and Roche-lobe overflow. WRLOF has an effect on the accretion efficiency of mass transfer and on the angular momentum lost by the binary system. The aim of this work is to understand the overall effect of WRLOF on the population of CEMP stars. To simulate populations of low-metallicity binaries we combined a synthetic nucleosynthesis model with a binary population synthesis code. In this code we implemented the WRLOF mechanism. We used the results of hydrodynamical simulations to model the effect of WRLOF on the accretion efficiency, and we took the effect on the angular momentum loss into account by assuming a simple prescription. The combination of these two effects widens the range of systems that become CEMP stars towards longer initial orbital periods and lower mass secondary stars. As a consequence the number of CEMP stars predicted by our model increases by a factor 1.2−1.8 compared to earlier results that consider the BHL prescription. Moreover, higher enrichments of carbon are produced, and the final orbital period distribution is shifted towards shorter periods.

Context. Extremely metal-poor (EMP) stars provide us with indirect information on the first generations of massive stars. The TOPoS survey has been designed to increase the census of these stars and to provide a chemical inventory that is as detailed as possible. Aims. Seven of the most iron-poor stars have been observed with the UVES spectrograph at the ESO VLT Kueyen 8.2 m telescope to refine their chemical composition. Methods. We analysed the spectra based on 1D LTE model atmospheres, but also used 3D hydrodynamical simulations of stellar atmospheres. Results. We measured carbon in six of the seven stars: all are carbon-enhanced and belong to the low-carbon band, defined in the TOPoS II paper. We measured lithium (A(Li) = 1.9) in the most iron-poor star (SDSS J1035+0641, [Fe/H] <−5.2). We were also able to measure Li in three stars at [Fe/H] ~−4.0, two of which lie on the Spite plateau. We confirm that SDSS J1349+1407 is extremely rich in Mg, but not in Ca. It is also very rich in Na. Several of our stars are characterised by low α-to-iron ratios. Conclusions. The lack of high-carbon band stars at low metallicity can be understood in terms of evolutionary timescales of binary systems. The detection of Li in SDSS J1035+0641 places a strong constraint on theories that aim at solving the cosmological lithium problem. The Li abundance of the two warmer stars at [Fe/H] ~−4.0 places them on the Spite plateau, while the third, cooler star, lies below. We argue that this suggests that the temperature at which Li depletion begins increases with decreasing [Fe/H]. SDSS J1349+1407 may belong to a class of Mg-rich EMP stars. We cannot assess if there is a scatter in α-to-iron ratios among the EMP stars or if there are several discrete populations. However, the existence of stars with low α-to-iron ratios is supported by our observations.

CEMP-r/s stars are metal-poor stars with enhanced abundances of carbon and heavy elements associated with the slow and rapid neutron-capture process (s-and r-elements, respectively). It is believed that carbon and s-elements were accreted in the past from the wind of a primary star in the asymptotic giant branch (AGB) phase of evolution, a scenario that is generally accepted to explain the formation of CEMP stars that are only enhanced in s-elements (CEMP-s stars). The origin of r-element-enrichment in CEMP-r/s stars is currently debated and many formation scenarios have been put forward. We aim to determine the likelihood of the scenarios proposed to explain the formation of CEMP-r/s stars. We calculate the frequency of CEMP-r/s stars among CEMP-s stars for a variety of formation scenarios, and we compare it with that determined from an observed sample of CEMP-r/s stars collected from the literature. The theoretical frequency of CEMP-r/s stars predicted in most formation scenarios underestimates the observed ratio by at least a factor of five. If the enrichments in s-and r-elements are independent, the model ratio of CEMP-r/s to CEMP-s stars is about 22%, that is approximately consistent with the lowest estimate of the observed ratio. However, this model predicts that about one third of all carbon-normal stars have [Ba/Fe] and [Eu/Fe] higher than one, and that 40% of all CEMP stars have [Ba/Eu] ≤ 0. Stars with these properties are at least ten times rarer in our observed sample. The intermediate or i-process, which is supposedly active in some circumstances during the AGB phase, could provide an explanation of the origin of CEMP-r/s stars, similar to that of CEMP-s stars, in the context of wind mass accretion in binary systems. Further calculations of the nucleosynthesis of the i-process and of the detailed evolution of late AGB stars are needed to investigate if this scenario predicts a CEMP-r/s star frequency consistent with the observations.

Many of the carbon-enhanced metal-poor (CEMP) stars that we observe in the Galactic halo are found in binary systems and show enhanced abundances of elements produced by the slow neutron-capture process (s-elements). The origin of the peculiar chemical abundances of these CEMP-s stars is believed to be accretion in the past of enriched material from a primary star in the asymptotic giant branch (AGB) phase of its evolution. We investigate the mechanism of mass transfer and the process of nucleosynthesis in low-metallicity AGB stars by modelling the binary systems in which the observed CEMP-s stars were formed. For this purpose we compare a sample of 67 CEMP-s stars with a grid of binary stars generated by our binary evolution and nucleosynthesis model. We classify our sample CEMP-s stars in three groups based on the observed abundance of europium. In CEMP-s/r stars the europium-toiron ratio is more than ten times higher than in the Sun, whereas it is lower than this threshold in CEMP-s/nr stars. No measurement of europium is currently available for CEMP-s/ur stars. On average our models reproduce the abundances observed in CEMP-s/nr stars well, whereas in CEMP-s/r stars and CEMP-s/ur stars the abundances of the light-s elements (strontium, yttrium, zirconium) are systematically overpredicted by our models, and in CEMP-s/r stars the abundances of the heavy-s elements (barium, lanthanum) are underestimated. In all stars our modelled abundances of sodium overestimate the observations. This discrepancy is reduced only in models that underestimate the abundances of most of the s-elements. Furthermore, the abundance of lead is underpredicted in most of our model stars, independent of the metallicity. These results point to the limitations of our AGB nucleosynthesis model, particularly in the predictions of the element-to-element ratios. In our models CEMP-s stars are typically formed in wide systems with periods above 10 000 days, while most of the observed CEMP-s stars are found in relatively close orbits with periods below 5000 days. This evidence suggests that either the sample of CEMP-s binary stars with known orbital parameters is biased towards short periods or that our wind mass-transfer model requires more efficient accretion in close orbits.

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, δ Si −0.27 ± 0.16 dex, is surprisingly close to that of iron, δ Fe −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 stellar population in the Galactic halo is characterised by a large fraction of carbon-enhanced metal-poor (CEMP) stars. Most CEMP stars have enhanced abundances of s-process elements (CEMP-s stars), and some of these are also enriched in r-process elements (CEMP-s/r stars). In one formation scenario proposed for CEMP stars, the observed carbon excess is explained by invoking wind mass transfer in the past from a more massive thermally-pulsing asymptotic giant branch (AGB) primary star in a binary system. In this work we generate synthetic populations of binary stars at metallicity Z = 0.0001 ([Fe/H] ≈ −2.3), with the aim of reproducing the observed fraction of CEMP stars in the halo. In addition, we aim to constrain our model of the wind masstransfer process, in particular the wind-accretion efficiency and angular-momentum loss, and investigate under which conditions our model populations reproduce observed distributions of element abundances. We compare the CEMP fractions determined from our synthetic populations and the abundance distributions of many elements with observations. Several physical parameters of the binary stellar population of the halo are uncertain, in particular the initial mass function, the mass-ratio distribution, the orbital-period distribution, and the binary fraction. We vary the assumptions in our model about these parameters, as well as the wind mass-transfer process, and study the consequent variations of our synthetic CEMP population. The CEMP fractions calculated in our synthetic populations vary between 7% and 17%, a range consistent with the CEMP fractions among very metal-poor stars recently derived from the SDSS/SEGUE data sample. The resulting fractions are more than a factor of three higher than those determined with default assumptions in previous population-synthesis studies, which typically underestimated the observed CEMP fraction. We find that most CEMP stars in our simulations are formed in binary systems with periods longer than 10 000 days. Few CEMP stars have measured orbital periods, but all that do have periods up to a few thousand days. Our results are consistent only if this small subpopulation represents the short-period tail of the underlying period distribution. The results of our comparison between the modelled and observed abundance distributions are significantly different for CEMP-s/r stars and for CEMP-s stars without strong enrichment in r-process elements. For the latter, our simulations qualitatively reproduce the observed distributions of carbon, sodium, and heavy elements such as strontium, barium, europium, and lead. Contrarily, for CEMP-s/r stars our model cannot reproduce the large abundances of neutron-rich elements such as barium, europium, and lead. This result is consistent with previous studies, and suggests that CEMP-s/r stars experienced a different nucleosynthesis history to CEMP-s stars.

AGB stars are responsible for producing a variety of elements, including carbon, nitrogen, and the heavy elements produced in the slow neutron-capture process (s-elements). There are many uncertainties involved in modelling the evolution and nucleosynthesis of AGB stars, and this is especially the case at low metallicity, where most of the stars with high enough masses to enter the AGB have evolved to become white dwarfs and can no longer be observed. The stellar population in the Galactic halo is of low mass ( 0.85 M ) and only a few observed stars have evolved beyond the first giant branch. However, we have evidence that low-metallicity AGB stars in binary systems have interacted with their low-mass secondary companions in the past. The aim of this work is to investigate AGB nucleosynthesis at low metallicity by studying the surface abundances of chemically peculiar very metal-poor stars of the halo observed in binary systems. To this end we select a sample of 15 carbon-and s-element-enhanced metal-poor (CEMP-s) halo stars that are found in binary systems with measured orbital periods. With our model of binary evolution and AGB nucleosynthesis, we determine the binary configuration that best reproduces, at the same time, the observed orbital period and surface abundances of each star of the sample. The observed periods provide tight constraints on our model of wind mass transfer in binary stars, while the comparison with the observed abundances tests our model of AGB nucleosynthesis. For most of the stars in our sample, we find that an episode of efficient wind mass transfer, combined with strong angular momentum loss, has occurred in the past. In some cases we find discrepancies between the observed and modelled abundances even if we adopt a fine-tuned set of parameters in our binary evolution model. These discrepancies are probably caused by missing physical ingredients in our models of AGB nucleosynthesis and they provide indications of how to improve our knowledge of the process of nucleosynthesis in AGB stars.

A large fraction of stars in binary systems are expected to undergo mass and angular momentum exchange at some point in their evolution, which can drastically alter the chemical and dynamical properties and fates of the systems. Interaction by stellar wind is an important process in wide binaries. However, the details of wind mass transfer are still not well understood. We perform threedimensional hydrodynamical simulations of wind mass transfer in binary systems to explore mass accretion efficiencies and geometries of mass outflows, for a range of mass ratios from 0.05 to 1.0. In particular, we focus on the case of a free wind, in which some physical mechanism accelerates the expelled wind material balancing the gravity of the mass-losing star with the wind velocity comparable to the orbital velocity of the system. We find that the mass accretion efficiency and accreted specific angular momentum increase with the mass ratio of the system. For an adiabatic wind, we obtain that the accretion efficiency onto the secondary star varies from about 0.1% to 8% for mass ratios between 0.05 and 1.0.