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About half of the stars in our Galaxy are born with a companion forming so-called binary systems. In these systems, the two stellar components are gravitationally bound and orbit each other during their evolution, which can be strongly affected by interaction with the companion, especially when one of the stars evolves to giant dimensions. Binary interactions also affect the evolution of the period and the eccentricity of the orbit. Many aspects of interaction physics in binaries are not understood yet, and investigating the products that result from interacting systems is crucial to unravelthe physical mechanisms involved. These statements are applicable to binaries of all masses, but this thesis focuses on low- and intermediate-mass systems, with components with initial masses between one and five times the mass of the Sun (Msun).Among the prototypical families of post-interaction binary systems in this mass range, one must mention Barium (Ba) stars. Ba stars are main-sequence or giant stars which show an enhancement of chemical elements that should not yet be overabundant at these evolutionary stages. Currently it is widely accepted that these chemicals were transferred from a more evolved companion during a phase of mass transfer in the system, and that this companion evolved into a cool white dwarf, which is too dim to be directly detected. However, the motion of the Ba star and its white-dwarf companion orbiting each other is detectable, and understanding the observed orbital properties of Ba-star systems, as well as the stellar properties of the Ba star and its polluter, is a key to the system’s interaction history. Moreover, identifying the mechanisms that impact the formation of Ba stars will teach us about binary evolution and interaction physics.This thesis presents observational and theoretical efforts made to contribute to a better understanding of the properties of Ba stars. The beginning of this work was well timed with the first data release of the G, Près de la moitié des étoiles de notre Galaxie naissent avec un compagnon et sont amenées à interagir durant leur évolution via des processus encore mal compris. En étudiant des systèmes d’étoiles doubles qui ont subit des interactions, nous pourrons découvrir les mécanismes physiques impliqués. Dans cette thèse, cette stratégie est appliquée pour étudier une certaine classe de systèmes binaires dont font partie les étoiles à baryum (Ba). Les étoiles Ba sont des étoiles géantes ou de la séquence principale (Ba naines) qui sont caractérisées par une sur-abondance en éléments chimiques, tel le baryum, qui ne peut provenir de l'étoile elle-même. Cette pollution chimique résulte d'un transfert de matière antérieur entre l'étoile initialement la plus massive et son compagnon. L'étoile à l’origine de la pollution est maintenant au stage naine blanche. L'analyse des paramètres orbitaux des étoiles à Ba et des propriétés des composantes stellaires, est une clé pour comprendre l’histoire de ces interactions.Cette thèse présente les efforts observationnels et théoriques apportés pour atteindre ce but. Cette thèse exploite la synergie entre les données du satellite Gaïa mise à disposition au début de ce travail, la spectroscopie haute résolution, les programmes de surveillance binaire à long terme et les modèles d'évolution stellaire et binaire de pointe.Dans la première partie, nous décrivons la méthodologie utilisée pour localiser avec précision grâce aux données Gaïa, les étoiles à Ba dans le diagramme de Hertzsprung-Russell et ainsi déterminer leur statut évolutif et leur masse. Nous montrons notamment que les géantes ont des masses variants entre 1 et 5 Msun avec une distribution piquée à 2 Msun.La deuxième partie présente la plus grande étude systématique des propriétés stellaires et orbitales des étoiles à Ba naine et une comparaison approfondie de ces propriétés avec celles des étoiles géantes. Les étoiles Ba de la séquence principale montrent des périodes orbitales co, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

Recent measurements of the nuclear level densities and γ-ray strength functions below the neutron thresholds in 180,181,182Ta are used as input in the nuclear reaction code TALYS. These experimental average quantities are utilized in the calculations of the 179,180,181Ta radiative neutron capture cross sections. From the latter, astrophysical Maxwellian-averaged (n,γ) cross sections (MACS) and reaction rates are extracted, which in turn are used in large astrophysical network calculations to probe the production mechanism of 180 Ta. These calculations are performed for two scenarios, the s-process production of 180,181Ta in Asymptotic Giant Branch (AGB) stars and the p-process nucleosynthesis of 180Tam in Type-II supernovae. Based on the results from this work, the s-process in stellar evolution is considered negligible in the production of 180Tam whereas 181Ta is partially produced by AGB stars. The new measurements strongly constrain the production and destruction rates of 180Tam at p-process temperatures and confirm significant production of nature’s rarest stable isotope 180Tam by the p-process., SCOPUS: ar.j, info:eu-repo/semantics/published

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Barium (Ba) dwarfs and CH subgiants are the less evolved analogues of Ba and CH giants. They are F-to G-Type main-sequence stars polluted with heavy elements by their binary companions when the companion was on the asymptotic giant branch (AGB). This companion is now a white dwarf that in most cases cannot be directly detected. We present a large systematic study of 60 objects classified as Ba dwarfs or CH subgiants. Combining radial-velocity measurements from HERMES and SALT high-resolution spectra with radial-velocity data from CORAVEL and CORALIE, we determine the orbital parameters of 27 systems. We also derive their masses by comparing their location in the Hertzsprung-Russell diagram with evolutionary models. We confirm that Ba dwarfs and CH subgiants are not at different evolutionary stages, and that they have similar metallicities, despite their different names. Additionally, Ba giants appear significantly more massive than their main-sequence analogues. This is likely due to observational biases against the detection of hotter main-sequence post-mass-Transfer objects. Combining our spectroscopic orbits with the HIPPARCOS astrometric data, we derive the orbital inclination and the mass of the WD companion for four systems. Since this cannot be done for all systems in our sample yet (but should be possible with upcoming Gaia data releases), we also analyse the mass-function distribution of our binaries. We can model this distribution with very narrow mass distributions for the two components and random orbital orientations on the sky. Finally, based on BINSTAR evolutionary models, we suggest that the orbital evolution of low-mass Ba systems can be affected by a second phase of interactions along the red giant branch of the Ba star, which impact the eccentricities and periods of the giants., SCOPUS: ar.j, info:eu-repo/semantics/published

Context. S stars are late-type giants with spectra showing characteristic molecular bands of ZrO in addition to the TiO bands typical of M stars. Their overabundance pattern shows the signature of s-process nucleosynthesis. Intrinsic, technetium (Tc)-rich S stars are the first objects on the asymptotic giant branch (AGB) to undergo third dredge-up (TDU) events. Exquisite Gaia parallaxes now allow for these stars to be precisely located in the Hertzsprung-Russell (HR) diagram. Here we report on a population of low-mass, Tc-rich S stars previously unaccounted for by stellar evolution models. Aims. Our aim is to derive parameters for a sample of low-mass, Tc-rich S stars and then, by comparing their location in the HR diagram with stellar evolution tracks, to derive their masses and to compare their measured s-process abundance profiles with recently derived STAREVOL nucleosynthetic predictions for low-mass AGB stars. Methods. Stellar parameters were obtained using a combination of HERMES high-resolution spectra, accurate Gaia Data Release 2 (Gaia-DR2) parallaxes, stellar-evolution models, and newly designed MARCS model atmospheres for S-type stars. Results. We report on six Tc-rich S stars lying close to the 1 M ⊙ (initial mass) tracks of AGB stars of the corresponding metallicity and above the predicted onset of TDU, as expected. This provides direct evidence for TDUs occurring in AGB stars with initial masses as low as ∼1 M⊙ and at low luminosity, that is, at the start of the thermally pulsing AGB. We present AGB models producing TDU in those stars with [Fe/H] in the range -0.25 to -0.5. There is reasonable agreement between the measured and predicted s-process abundance profiles. For two objects however, CD -29°5912 and BD +34°1698, the predicted C/O ratio and s-process enhancements do not simultaneously match the measured ones., SCOPUS: ar.j, info:eu-repo/semantics/published

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Context. Barium stars are s-process enriched giants. They owe their chemical peculiarities to a past mass transfer phase. During this phase they were polluted by their binary companion, which at the time was an asymptotic giant branch (AGB) star, but is now an extinct white dwarf. Barium stars are thus ideal targets for understanding and constraining the s-process in low-and intermediate-mass AGB stars. Aims. We derive the abundances of a large number of heavy elements in order to shed light on the conditions of operation of the neutron source responsible for the production of s-elements in the former companions of the barium stars. Methods. Adopting a recently used methodology, we analyse a sample of eighteen highly enriched barium stars observed with the high-resolution HERMES spectrograph mounted on the Mercator telescope (La Palma). We determine the stellar parameters and abundances using MARCS model atmospheres. In particular, we derive the Nb-Zr ratio which was previously shown to be a sensitive thermometer for the s-process nucleosynthesis. Indeed, in barium stars, 93Zr has fully decayed into mono-isotopic 93Nb, so Nb/Zr is a measure of the temperature-sensitive 93Zr/Zr isotopic ratio. Results. HD 28159, previously classified as K5III and initially selected to serve as a reference cool K star for our abundance analysis, turns out to be enriched in s-process elements, and as such is a new barium star. Four stars are characterised by high nitrogen abundances, and among those three have high [Nb/Zr] and [hs/ls] ratios. The derived Zr and Nb abundances provide more accurate constraints on the s-process neutron source, identified to be 13C(α, n)16O for barium stars. The comparison with stellar evolution and nucleosynthesis models shows that the investigated barium stars were polluted by a low-mass (M ∼ 2-3 M⊙) AGB star. HD 100503 is potentially identified as a high metallicity analogue of carbon-enhanced metal-poor star enriched in both r-and s-process elements (C, SCOPUS: ar.j, info:eu-repo/semantics/published

Getting a better understanding of the evolution and nucleosynthetic yields of the most metal-poor stars ( Z ¿ 10 ¿5 ) is critical because they are part of the big picture of the history of the primitive universe. Yet many of the remaining unknowns of stellar evolution lie in the birth, life, and death of these objects. We review stellar evolution of intermediate-mass Z = 10 ¿5 models existing in the literature, with a particular focus on the problem of their final fates. We emphasise the importance of the mixing episodes between the stellar envelope and the nuclearly processed core, which occur after stars exhaust their central He (second dredge-up and dredge-out episodes). The depth and efficiency of these episodes are critical to determine the mass limits for the formation of electron-capture SNe. Our knowledge of these phenomena is not complete because they are strongly affected by the choice of input physics. These uncertainties affect stars in all mass and metallicity ranges. However, difficulties in calibration pose additional challenges in the case of the most metal-poor stars. We also consider the alternative SN I1/2 channel to form SNe out of the most metal-poor intermediate-mass objects. In this case, it is critical to understand the thermally pulsing Asymptotic Giant Branch evolution until the late stages. Efficient second dredge-up and, later, third dredge-up episodes could be able to pollute stellar envelopes enough for the stars to undergo thermal pulses in a way very similar to that of higher initial Z objects. Inefficient second and/or third dredge-up may leave an almost pristine envelope, unable to sustain strong stellar winds. This may allow the H-exhausted core to grow to the Chandrasekhar mass before the envelope is completely lost, and thus let the star explode as an SN I1/2. After reviewing the information available on these two possible channels for the formation of SNe, we discuss existing nucleosynthetic yields of stars of metallicity Z = 10, Peer Reviewed, Postprint (author's final draft)

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Context. S-process elements are observed at the surface of low- and intermediate-mass stars. These observations can be explained empirically by the so-called partial mixing of protons scenario leading to the incomplete operation of the CN cycle and a significant primary production of the neutron source. This scenario has been successful in qualitatively explaining the s-process enrichment in AGB stars. Even so, it remains difficult to describe both physically and numerically the mixing mechanisms taking place at the time of the third dredged-up between the convective envelope and the underlying C-rich radiative layer Aims. We aim to present new calculations of the s-process nucleosynthesis in AGB stars testing two different numerical implementations of chemical transport. These are based on a diffusion equation which depends on the second derivative of the composition and on a numerical algorithm where the transport of species depends linearly on the chemical gradient. Methods. The s-process nucleosynthesis resulting from these different mixing schemes is calculated with our stellar evolution code STAREVOL which has been upgraded to include an extended s-process network of 411 nuclei. Our investigation focuses on a fiducial 2 M⊙, [Fe/H] = -0.5 model star, but also includes four additional stars of different masses and metallicities. Results. We show that for the same set of parameters, the linear mixing approach produces a much larger 13C-pocket and consequently a substantially higher surface s-process enrichment compared to the diffusive prescription. Within the diffusive model, a quite extreme choice of parameters is required to account for surface s-process enrichment of 1-2 dex. These extreme conditions can not, however, be excluded at this stage. Conclusions. Both the diffusive and linear prescriptions of the overshoot mixing are suited to describe the s-process nucleosynthesis in AGB stars provided the profile of the diffusion coefficient below the convective, SCOPUS: ar.j, info:eu-repo/semantics/published

Context. TZ Fornacis (TZ For) is an evolved detached binary system that is difficult to model and interpret, but very useful for testing stellar evolution theory and physics. Aims. We aim to search for solutions that are self-consistent and to determine the necessary stellar physics input. We also check solutions found previously for their internal consistency and for reproducibility. Methods. We use both a single and a binary stellar evolution code, and take into account all known system properties. We determine the physical stellar parameters by imposing that the models match the known radii for identical stellar ages. The evolution has to be consistent with a binary system in classical Roche geometry. Results. We obtained two different solutions to model TZ For successfully. Both depend on avoiding a long evolution on the first giant branch and imply a sufficiently large convective core on the main sequence. TZ For can be modelled consistently as a detached binary system by invoking either a substantial amount of core overshooting or a tidally enhanced wind mass loss along the red giant branch. An evolution with Roche-lobe overflow can definitely be excluded. Conclusions. A comparison of our results with previous studies also reveals that in addition to uncertainties associated with the input physics, the modelling of overshooting by different algorithms can have a strong impact., SCOPUS: ar.j, info:eu-repo/semantics/published

Context. Most super-asymptotic giant branch (SAGB) stars are expected to end their life as oxygen-neon white dwarfs rather than electron capture supernovae (ECSN). The reason is ascribed to the ability of the second dredge-up to significantly reduce the mass of the He core and of the efficient AGB winds to remove the stellar envelope before the degenerate core reaches the critical mass for the activation of electron capture reactions. Aims. In this study, we investigate the formation of ECSN through case A and case B mass transfer. In these scenarios, when Roche lobe overflow stops, the primary has become a helium star. With a small envelope left, the second dredge-up is prevented, potentially opening new paths to ECSN. Methods. We compute binary models using our stellar evolution code BINSTAR. We consider three different secondary masses of 8, 9, and 10 M⊙ and explore the parameter space, varying the companion mass, orbital period, and input physics. Results. Assuming conservative mass transfer, with our choice of secondary masses all case A systems enter contact either during the main sequence or as a consequence of reversed mass transfer when the secondary overtakes its companion during core helium burning. Case B systems are able to produce ECSN progenitors in a relatively small range of periods (3 ≲ P(d) ≤ 30) and primary masses (10:9 ≤ M= M⊙ ≤ 11:5). Changing the companion mass has little impact on the primary's fate as long as the mass ratio M1=M2 remains less than 1.4-1.5, above which evolution to contact becomes unavoidable. We also find that allowing for systemic mass loss substantially increases the period interval over which ECSN can occur. This change in the binary physics does not however affect the primary mass range. We finally stress that the formation of ECSN progenitors through case A and B mass transfer is very sensitive to adopted binary and stellar physics. Conclusions. Close binaries provide additional channels for ECSN but the parameter space, SCOPUS: ar.j, info:eu-repo/semantics/published

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We noticed that the values of P and to a lower extent P shown in Fig. 17 of our article Escorza et al. (2017) had been wrongly estimated because the algorithm to find the maximum stellar radius contained an error. In the original plot, we did not discard the core-He burning phase and for stars (Figure Presented)., SCOPUS: er.j, info:eu-repo/semantics/published

Due to the second dredge-up and expected strong mass loss during the thermally pulsing super-AGB phase, the mass range for single stars to evolve as electron capture supernova (ECSN) is very narrow. In this short contribution, we briefly review alternative binary channels and present recent case A & B mass transfer simulations. In these models, the envelope is removed during Roche lobe overflow (RLOF), preventing the occurrence of the second dredge-up and the reduction of the H-free core below the Chandrasekhar mass. The newly formed helium star can then ignite carbon and may end its life as ECSN., SCOPUS: cp.p, info:eu-repo/semantics/published

With the availability of parallaxes provided by the Tycho-Gaia Astrometric Solution, it is possible to construct the Hertzsprung-Russell diagram (HRD) of barium and related stars with unprecedented accuracy. A direct result from the derived HRD is that subgiant CH stars occupy the same region as barium dwarfs, contrary to what their designations imply. By comparing the position of barium stars in the HRD with STAREVOL evolutionary tracks, it is possible to evaluate their masses, provided the metallicity is known. We used an average metallicity [Fe/H] = -0.25 and derived the mass distribution of barium giants. The distribution peaks around 2.5 M with a tail at higher masses up to 4.5 M. This peak is also seen in the mass distribution of a sample of normal K and M giants used for comparison and is associated with stars located in the red clump. When we compare these mass distributions, we see a deficit of low-mass (1 - 2 M) barium giants. This is probably because low-mass stars reach large radii at the tip of the red giant branch, which may have resulted in an early binary interaction. Among barium giants, the high-mass tail is however dominated by stars with barium indices of less than unity, based on a visual inspection of the barium spectral line; that is, these stars have a very moderate barium line strength. We believe that these stars are not genuine barium giants, but rather bright giants, or supergiants, where the barium lines are strengthened because of a positive luminosity effect. Moreover, contrary to previous claims, we do not see differences between the mass distributions of mild and strong barium giants., SCOPUS: ar.j, info:eu-repo/semantics/published

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We review the lives, deaths and nucleosynthetic signatures of intermediate-mass stars in the range ˜6–12 M¿, which form super-AGB stars near the end of their lives. The critical mass boundaries both between different types of massive white dwarfs (CO, CO–Ne, ONe), and between white dwarfs and supernovae, are examined along with the relative fraction of super-AGB stars that end life either as an ONe white dwarf or as a neutron star (or an ONeFe white dwarf), after undergoing an electron capture supernova event. The contribution of the other potential single-star channel to electron-capture supernovae, that of the failed massive stars, is also discussed. The factors that influence these different final fates and mass limits, such as composition, rotation, the efficiency of convection, the nuclear reaction rates, mass-loss rates, and third dredge-up efficiency, are described. We stress the importance of the binary evolution channels for producing electron-capture supernovae. Recent nucleosynthesis calculations and elemental yield results are discussed and a new set of s-process heavy element yields is presented. The contribution of super-AGB star nucleosynthesis is assessed within a Galactic perspective, and the (super-)AGB scenario is considered in the context of the multiple stellar populations seen in globular clusters. A brief summary of recent works on dust production is included. Last, we conclude with a discussion of the observational constraints and potential future advances for study into these stars on the low mass/high mass star boundary., Peer Reviewed, Postprint (author's final draft)

Context. This paper is part of a larger project in which we systematically study the chemical abundances of Galactic and extragalactic post-asymptotic giant branch (post-AGB) stars. The goal at large is to provide improved observational constraints to the models of the complex interplay between the AGB s-process nucleosynthesis and the associated mixing processes. Aims. Lead (Pb) is the final product of the s-process nucleosynthesis and is predicted to have large overabundances with respect to other s-process elements in AGB stars of low metallicities. However, Pb abundance studies of s-process enriched post-AGB stars in the Magellanic Clouds show a discrepancy between observed and predicted Pb abundances. The determined upper limits based on spectral studies are much lower than what is predicted. In this paper, we focus specifically on the Pb abundance of 14 Galactic s-process enhanced post-AGB stars to check whether the same discrepancy is present in the Galaxy as well. Among these 14 objects, two were not yet subject to a detailed abundance study in the literature. We apply the same method to obtain accurate abundances for the 12 others. Our homogeneous abundance results provide the input of detailed spectral synthesis computations in the spectral regions where Pb lines are located. Methods. We used high-resolution UVES and HERMES spectra for detailed spectral abundance studies of our sample of Galactic post-AGB stars. None of the sample stars display clear Pb lines, and we only deduced upper limits of the Pb abundance by using spectrum synthesis in the spectral ranges of the strongest Pb lines. Results. We do not find any clear evidence of Pb overabundances in our sample. The derived upper limits are strongly correlated with the effective temperature of the stars with increasing upper limits for increasing effective temperatures. We obtain stronger Pb constraints on the cooler objects. Moreover, we confirm the s-process enrichment and carbon enhancement of two u, SCOPUS: ar.j, info:eu-repo/semantics/published

A large fraction of the carbon-enhanced, extremely metal-poor halo giants ([Fe/H] <-2.5) are also strongly enriched in neutron-capture elements from the s process (CEMP-s stars). The conventional explanation for the properties of these stars is mass transfer from a nearby binary companion on the asymptotic giant branch (AGB). This scenario leads to a number of testable predictions in terms of the properties of the putative binary system and the resulting abundance pattern. Among the CEMP stars, some stars further exhibit overabundances in r-process elements on top of the s-process enrichment, and are tagged CEMP-rs stars. Although the nucleosynthesis process responsible for this kind of mixed abundance pattern is still under debate, CEMP-rs stars seem to belong to binary systems as do CEMP-s stars. Aims. Our aim is to present and analyse in detail our comprehensive data set of systematic radial-velocity measurements and high-resolution spectroscopy of the CEMP star HE 0017+0055. Methods. Our precise radial-velocity monitoring of HE 0017+0055 over 2940 days (8 yr) with the Nordic Optical Telescope and Mercator telescopes exhibits variability, with a period of 384 d and amplitude of 540 ± 27 m s-1 superimposed on a nearly linear long-term decline of ~1 m s-1 day-1. We used high-resolution HERMES/Mercator and Keck/HIRES spectra to derive elemental abundances with 1D LTE MARCS models. A metallicity of [Fe/H] ~-2.4 is found, along with s-process overabundances of the order of 2 dex (with the exception of [Y/Fe] ~ +0.5), and most notably overabundances of r-process elements like Sm, Eu, Dy, and Er in the range 0.9-2.0 dex. With [Ba/Fe] > 1.9 dex and [Eu/Fe] = 2.3 dex, HE 0017+0055 is a CEMP-rs star. We used the derived atmospheric parameters and abundances to infer HE 0017+0055 evolutionary status from a comparison with evolutionary tracks. Results. HE 0017+0055 appears to be a giant star below the tip of the red giant branch. The s-process pollution must therefore origin, SCOPUS: ar.j, info:eu-repo/semantics/published

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The theoretical and observed populations of pre-cataclysmic variables are dominated by systems with low-mass white dwarfs (WDs), while the WD masses in cataclysmic variables (CVs) are typically high. In addition, the space density of CVs is found to be significantly lower than in the theoretical models. We investigate the influence of nova outbursts on the formation and initial evolution of CVs. In particular, we calculate the stability of the mass transfer in the case where all of the material accreted on the WD is lost in classical novae and part of the energy to eject the material comes from a common-envelope-like interaction with the companion. In addition, we study the effect of an asymmetry in the mass ejection that may lead to small eccentricities in the orbit. We find that a common-envelope-like ejection significantly decreases the stability of the mass transfer, particularly for low-mass WDs. Similarly, the influence of asymmetric mass loss can be important for short-period systems and even more so for low-mass WDs; however, this influence likely disappears long before the next nova outburst due to orbital circularization. In both cases the mass-transfer rates increase, which may lead to observable (and perhaps already observed) consequences for systems that do survive to become CVs. However, a more detailed investigation of the interaction between nova ejecta and the companion and the evolution of slightly eccentric CVs is needed before definite conclusions can be drawn., SCOPUS: ar.j, info:eu-repo/semantics/published

Roughly half of the heavy elements (atomic mass greater than that of iron) are believed to be synthesized in the late evolutionary stages of stars with masses between 0.8 and 8 solar masses. Deep inside the star, nuclei (mainly iron) capture neutrons and progressively build up (through the slow-neutron-capture process, or s-process) heavier elements that are subsequently brought to the stellar surface by convection. Two neutron sources, activated at distinct temperatures, have been proposed: 13C and 22Ne, each releasing one neutron per α-particle (4He) captured. To explain the measured stellar abundances, stellar evolution models invoking the 13C neutron source (which operates at temperatures of about one hundred million kelvin) are favoured. Isotopic ratios in primitive meteorites, however, reflecting nucleosynthesis in the previous generations of stars that contributed material to the Solar System, point to higher temperatures (more than three hundred million kelvin), requiring at least a late activation of 22Ne. Here we report a determination of the s-process temperature directly in evolved low-mass giant stars, using zirconium and niobium abundances, independently of stellar evolution models. The derived temperature supports 13C as the s-process neutron source. The radioactive pair 93Zr–93Nb used to estimate the s-process temperature also provides, together with the pair 99Tc–99Ru, chronometric information on the time elapsed since the start of the s-process, which we determine to be one million to three million years., http://www.nature.com/nature/journal/v517/n7533/full/nature14050.html, SCOPUS: ar.j, info:eu-repo/semantics/published

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In recent years much interest has been shown in the process of thermohaline mixing in red giants. In low- and intermediate-mass stars this mechanism first activates at the position of the bump in the luminosity function, and has been identified as a likely candidate for driving the slow mixing inferred to occur in these stars. One particularly important consequence of this process, which is driven by a molecular weight inversion, is the destruction of lithium. We show that the degree of lithium destruction, or in some cases production, is extremely sensitive to the numerical details of the stellar models. Within the standard 1D diffusion approximation to thermohaline mixing, we find that different evolution codes, with their default numerical schemes, can produce lithium abundances that differ from one another by many orders of magnitude. This disagreement is worse for faster mixing. We perform experiments with four independent stellar evolution codes, and derive conditions for the spatial and temporal resolution required for a converged numerical solution. The results are extremely sensitive to the time-steps used. We find that predicted lithium abundances published in the literature until now should be treated with caution., SCOPUS: ar.j, info:eu-repo/semantics/published