arXiv subjects

L. Siess

Publications and source records attributed to L. Siess.

At least 37 records · Page 2Linked to original sources

arXiv2023

Does the i-process operate at nearly solar metallicity?

A sample of 895 s-process-rich candidates has been found among the 454180 giant stars surveyed by LAMOST at low spectral resolution (R~1800). In a previous study, taking advantage of the higher resolution (R~86 000) offered by the the HERMES-Mercator spectrograph, we performed the re-analysis of 15 among the brightest stars of this sample. Among these 15 program stars, having close-to-solar metallicities, 11 showed mild to strong heavy element overabundances. The nucleosynthesis process(es) at the origin of these overabundances were however not questioned in our former study. We derive the abundances in s- and r-process elements of the 15 targets in order to investigate whether some stars also show an i-process signature, as sometimes found in their lower metallicity counterparts (namely, the Carbon-Enhanced Metal-Poor (CEMP)-rs stars). Abundances are derived from the high-resolution HERMES spectra for Pr, Nd, Sm, and Eu, using the TURBOSPECTRUM radiative transfer LTE code with MARCS model atmospheres. Using the new classification scheme proposed in our recent study we find that two stars show overabundances in both s- and r-process elements well above the level expected from the Galactic chemical evolution, an analogous situation to the one of CEMP-rs stars at lower metallicities. We compare the abundances of the most enriched stars with the nucleosynthetic predictions from the STAREVOL stellar evolutionary code and find abundances compatible with an i-process occurring in AGB stars. Despite a larger number of heavy elements to characterize the enrichment pattern, the limit between CEMP-s and CEMP-rs stars remains fuzzy. It is however interesting to note that an increasing number of extrinsic stars are found to have abundances better reproduced by an i-process pattern even at close-to-solar metallicities.

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arXiv2022

Synthesis of thorium and uranium in asymptotic giant branch stars

The intermediate neutron capture process (i-process) operates at neutron densities between those of the slow and rapid neutron-capture processes. It can be triggered by the ingestion of protons in a convective helium-burning region. One possible astrophysical site is low-mass low-metallicity asymptotic giant branch (AGB) stars. We study here the possibility that actinides (particularly Th and U) may be significantly synthesized through i-process nucleosynthesis in AGB stars. We computed a 1 $M_{\odot}$ model at [Fe/H] $= -2.5$ with the stellar evolution code STAREVOL. We used a nuclear network of 1160 species from H to Cf coupled to the transport processes. During the proton ingestion event, the neutron density goes up to $\sim 10^{15}$ cm$^{-3}$. While most of the nuclear flow cycles in the neutron-rich Pb-Bi-Po region, a non-negligible fraction leaks towards heavier elements and eventually synthesizes actinides. The surface enrichment in Th and U is subject to nuclear and astrophysical model uncertainties that could be lowered in the future, in particular by a detailed analysis of the nuclear inputs that affect the neutron capture rates of neutron-rich isotopes between Pb and Pa. One stellar candidate that may confirm the production of actinides by the i-process is the carbon-enhanced metal-poor r/s star J0949-1617, which shows Th lines in its spectrum. Its surface abundance is shown to be reasonably well reproduced by our AGB model. Combined with cosmochronometry, this finding opens the way to dating the i-process event and thus obtaining a lower limit for the age of CEMP-r/s stars. Such a dating is expected to be accurate only if surface abundances of Th and U can be extracted simultaneously. This work shows that actinides can be synthesized in AGB stars through the i-process. As a consequence, the r-process may not be the sole mechanism for the production of U and Th.

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arXiv2022

The intermediate neutron capture process. III. The i-process in AGB stars of different masses and metallicities without overshoot

Alongside the slow (s) and rapid (r) neutron capture processes, an intermediate neutron capture process (i-process) is thought to exist. It happens when protons are mixed in a convective helium-burning zone, and is referred to as proton ingestion event (PIE). A possible astrophysical site is the asymptotic giant branch (AGB) phase of low-mass low-metallicity stars. We provide i-process yields of a grid of AGB stars experiencing PIEs. We computed 12 models with initial masses of 1, 2, and 3 $M_{\odot}$ and metallicities of [Fe/H] $=-3.0$, $-2.5$ $-2.3,$ and $-2.0, $ with the stellar evolution code STAREVOL. We used a nuclear network of 1160 species at maximum, coupled to the chemical transport equations. These simulations do not include any extra mixing process. Proton ingestion takes place in six out of our 12 AGB models. These models experience i-process nucleosynthesis characterized by neutron densities of $\simeq 10^{14} -10^{15}$ cm$^{-3}$. Depending on the PIE properties two different evolution paths follow: either the stellar envelope is quickly lost and no more thermal pulses develop or the AGB phase resumes with additional thermal pulses. This behaviour critically depends on the pulse number when the PIE occurs, the mass of the ingested protons, and the extent to which the pulse material is diluted in the convective envelope. The surface enrichment after a PIE is a robust feature of our models and it persists under various convective assumptions. Our models can synthesise heavy elements up to Pb without any parametrized extra mixing process such as overshoot or inclusion of a $^{13}$C-pocket. Nevertheless, it remains to be explored how the i-process depends on mixing processes, such as overshoot, thermohaline, or rotation.

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arXiv2022

3D simulations of AGB stellar winds -- I. Steady winds and dust formation

Aims. We present the implementation of the treatment of particle ejection and dust nucleation in the smoothed particle hydrodynamics (SPH) code phantom. These developments represent the first step toward a more complete modeling of dust-driven winds emanating from AGB stars. Methods. The AGB outflow is modeled by injecting the SPH particles from a spherical inner boundary. This boundary is a series of concentric shells, with the AGB star at its center, and the particles are positioned on these shells on the vertices of an isocahedron geodesic surface. The outermost shell is ejected with a predefined radial velocity, and subsequent lower shells replenish the ejected ones, all rotated randomly to improve the isotropy of the outflow. The physical properties of the particles on these shells are set by solving the 1D analytic steady wind equations. The formation of dust is calculated starting from a compact chemical network for carbon-rich material, which creates the building blocks of the solid-state particles. Subsequently, the theory of the moments is used to obtain dust growth rates, without requiring knowledge on the grain size distribution. Results. We tested our implementation against a series of 1D reference solutions. We demonstrate that our method is able to reproduce Parker-type wind solutions. For the trans-sonic solution, small oscillations are present in the vicinity of the sonic point, but these do not impact the trans-sonic passage or terminal wind velocity. Supersonic solutions always compare nicely with 1D analytic profiles. We also tested our implementation of dust using two formalisms: an analytic prescription for the opacity devised by Bowen and the full treatment of carbon-dust formation. Both simulations reproduce the 1D analytic solution displaying the expected additional acceleration when the gas temperature falls below the condensation temperature.

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arXiv2022

The intermediate neutron capture process. I. Development of the i-process in low-metallicity low-mass AGB stars

Results from observations report a growing number of metal-poor stars showing an abundance pattern midway between the s- and r-processes. These so-called r/s-stars raise the need for an intermediate neutron capture process (i-process), which is thought to result from the ingestion of protons in a convective helium-burning region, but whose astrophysical site is still largely debated. We investigate whether an i-process during the asymptotic giant branch (AGB) phase of low-metallicity low-mass stars can develop and whether it can explain the abundances of observed r/s-stars. At the beginning of the AGB phase, during the third thermal pulse, the helium driven convection zone is able to penetrate the hydrogen-rich layers. The subsequent proton ingestion leads to a strong neutron burst with neutron densities of $\approx 4.3 \times 10^{14}$ cm$^{-3}$ at the origin of the synthesis of i-process elements. The nuclear energy released by proton burning in the helium-burning convective shell strongly affects the internal structure: the thermal pulse splits and after approximately ten years the upper part of the convection zone merges with the convective envelope. The surface carbon abundance is enhanced by more than 3 dex. This leads to an increase in the opacity, which triggers a strong mass loss and prevents any further thermal pulse. We show that specific isotopic ratios of Ba, Nd, Sm, and Eu can represent good tracers of i-process nucleosynthesis. Finally, an extended comparison with 14 selected r/s-stars show that the observed composition patterns can be well reproduced by our i-process AGB model.

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arXiv2021

The intermediate neutron capture process II.Nuclear uncertainties

Carbon-enhanced metal-poor (CEMP) r/s-stars show surface-abundance distributions characteristic of the so-called intermediate neutron capture process (i-process) of nucleosynthesis. We previously showed that the ingestion of protons in the convective helium-burning region of a low-mass low-metallicity star can explain the surface abundance distribution observed in CEMP r/s stars relatively well. Such an i-process requires detailed reaction network calculations involving hundreds of nuclei for which reaction rates have not yet been determined experimentally. We investigate the nuclear physics uncertainties affecting the i-process during the AGB phase of low-metallicity low-mass stars by propagating the theoretical uncertainties in the radiative neutron capture cross sections, as well as the 13C(a,n)16O reaction rate, and estimating their impact on the surface-abundance distribution. It is found that considering systematic uncertainties on the various nuclear ingredients affecting the radiative neutron capture rates, surface elemental abundances are typically predicted within +/-0.4 dex. The 56 < Z < 59 region of the spectroscopically relevant heavy-s elements of Ba-La-Ce-Pr as well as the r-dominated Eu element remain relatively unaffected by nuclear uncertainties. In contrast, the inclusion of the direct capture contribution impacts the rates in the neutron-rich A~45, 100, 160, and 200 regions, and the i-process production of the Z~45 and 65-70 elements. Uncertainties in the photon strength function also impact the overabundance factors by typically 0.2-0.4 dex. Nuclear level densities tend to affect abundance predictions mainly in the Z=74-79 regions. The uncertainties associated with the neutron-producing reaction 13C(a,n)16O and the unknown beta-decay rates are found to have a low impact on the overall surface enrichment

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arXiv2021

Sr and Ba abundance determinations: comparing machine-learning with star-by-star analyses -- High-resolution re-analysis of suspected LAMOST barium stars

A new large sample of 895 s-process-rich candidates out of 454180 giant stars surveyed by LAMOST at low spectral resolution (R ~ 1800) has been reported by Norfolk et al. (2019; hereafter N19). We aim at confirming the s-process enrichment at the higher resolution (R ~ 86000) offered by the HERMES-Mercator spectrograph, for the 15 brightest targets of the previous study sample which consists in 13 Sr-only stars and two Ba-only stars. Abundances were derived for elements Li, C, N, O, Na, Mg, Fe, Rb, Sr, Y, Zr, Nb, Ba, La, and Ce. Binarity has been tested by comparing the Gaia DR2 radial velocity with the HERMES velocity obtained 1600 - 1800 days later. Among the 15 programme stars, four show no s-process overabundances ([X/Fe] < 0.2 dex), eight show mild s-process overabundances (at least three heavy elements with 0.2 < [X/Fe] < 0.8), and three have strong overabundances (at least three heavy elements with [X/Fe] > 0.8). Among the 13 stars classified as Sr-only by the previous investigation, four have no s-process overabundances, eight are mild barium stars, and one is a strong barium star. The two Ba-only stars turn out to be both strong barium stars and are actually dwarf barium stars. They also show clear evidence for being binaries. Among the no-s stars, there are two binaries out of four, whereas only one out of the eight mild barium stars show a clear signature of radial-velocity variations. Blending effects and saturated lines have to be considered very carefully when using machine-learning techniques, especially on low-resolution spectra. Among the Sr-only stars from the previous study sample, one may expect about 60% (8/13) of them to be true mild barium stars and about 8% to be strong barium stars, and this fraction is likely close to 100% for the previous study Ba-only stars (2/2).

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arXiv2021

SPH modelling of wind-companion interactions in eccentric AGB binary systems

The late evolutionary stages of low- and intermediate-mass stars are characterised by mass loss through a dust-driven stellar wind. Recent observations reveal complex structures within these winds, that are believed to be formed primarily via interaction with a companion. How these complexities arise, and which structures are formed in which type of systems, is still poorly understood. Particularly, there is a lack of studies investigating the structure formation in eccentric systems. We aim to improve our understanding of the wind morphology of eccentric AGB binary systems by investigating the mechanism responsible for the different small-scale structures and global morphologies that arise in a polytropic wind with different velocities. Using the smoothed particle hydrodynamics (SPH) code Phantom, we generate nine different high-resolution, 3D simulations of an AGB star with a solar-mass companion with various wind velocity and eccentricity combinations. The models assume a polytropic gas, with no additional cooling. We conclude that for models with a high wind velocity, the short interaction with the companion results in a regular spiral morphology, that is flattened. In the case of a lower wind velocity, the stronger interaction results in the formation of a high-energy region and bow-shock structure that can shape the wind into an irregular morphology if instabilities arise. High-eccentricity models show a complex, phase-dependent interaction leading to wind structures that are irregular in three dimensions. However, the significant interaction with the companion compresses matter into an equatorial density enhancement, irrespective of eccentricity.

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arXiv2021

SPH modelling of companion-perturbed AGB outflows including a new morphology classification scheme

Asymptotic giant branch (AGB) stars are known to lose a significant amount of mass by a stellar wind, which controls the remainder of their stellar lifetime. High angular-resolution observations show that the winds of these cool stars typically exhibit mid- to small-scale density perturbations such as spirals and arcs, believed to be caused by the gravitational interaction with a (sub-)stellar companion. We aim to explore the effects of the wind-companion interaction on the 3D density and velocity distribution of the wind, as a function of three key parameters: wind velocity, binary separation and companion mass. For the first time, we compare the impact on the outflow of a planetary companion to that of a stellar companion. We intend to devise a morphology classification scheme based on a singular parameter. With our grid of models we cover the prominent morphology changes in a companion-perturbed AGB outflow: slow winds with a close, massive binary companion show a more complex morphology. Additionally, we prove that massive planets are able to significantly impact the density structure of an AGB wind. We find that the interaction with a companion affects the terminal velocity of the wind, which can be explained by the gravitational slingshot mechanism. We distinguish between two types of wind focussing to the orbital plane resulting from distinct mechanisms: global flattening of the outflow as a result of the AGB star's orbital motion and the formation of an EDE as a consequence of the companion's gravitational pull. We investigate different morphology classification schemes and uncover that the ratio of the gravitational potential energy density of the companion to the kinetic energy density of the AGB outflow yields a robust classification parameter for the models presented in this paper.

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arXiv2021

Low-mass low-metallicity AGB stars as an efficient i-process site explaining CEMP-rs stars

Among Carbon-Enhanced Metal-Poor (CEMP) stars, some are found to be enriched in s-process elements (CEMP-s), in r-process elements (CEMP-r) or in both s- and r-process elements (CEMP-rs). The origin of the abundance differences between CEMP-s and CEMP-rs stars is presently unknown. It has been claimed that the i-process, whose site still remains to be identified, could better reproduce CEMP-rs abundances than the s-process. We analyze high-resolution spectra of 25 metal-poor stars, observed with the high-resolution HERMES spectrograph mounted on the Mercator telescope, La Palma, or with the UVES/VLT and HIRES/KECK spectrographs. We propose a new, robust classification method for CEMP-s and CEMP-rs stars using eight heavy element abundances. The abundance profiles of CEMP-s and CEMP-rs stars are derived and there appears to be an abundance continuum between the two stellar classes. CEMP-rs stars present most of the characteristics of extrinsic stars such as CEMP-s, CH, Barium and extrinsic S stars, with an even larger binarity rate among CEMP-rs stars than among CEMP-s stars. Stellar evolutionary tracks of an enhanced carbon composition (consistent with our abundance determinations) are necessary to explain the position of CEMP-s and CEMP-rs stars in the HR diagram using Gaia DR2 parallaxes; they are found to lie mostly on the RGB. CEMP-rs stars can be explained as being polluted by a low-mass, low-metallicity TP-AGB companion experiencing i-process nucleosynthesis after proton ingestion during its first convective thermal pulses. The global fitting of our i-process models to CEMP-rs stars is as good as the one of our s-process models to CEMP-s stars. As such, CEMP-rs stars could be renamed as CEMP-sr stars, since they represent a particular manifestation of the s-process at low-metallicities. For these objects a call for an exotic i-process site may not necessarily be required anymore.

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arXiv2020

Planet formation in intermediate-separation binary systems

We report the first characterisation of the individual discs in the intermediate separation binary systems KK Oph and HD 144668 at millimetre wavelengths. In both systems the circum-primary and the circum-secondary discs are detected in the millimetre continuum emission, but not in $^{13}$CO nor C$^{18}$O lines. Even though the disc structure is only marginally resolved, we find indications of large-scale asymmetries in the outer regions of the primary discs, most likely due to perturbation by the companion. The derived dust masses are firmly above debris disc level for all stars. The primaries have about three times more dust in their discs than the secondaries. In the case of HD 144668 the opacity spectral index of the primary and secondary differ by the large margin of 0.69 which may be a consequence of the secondary disc being more compact. Upper limits on the gas masses imply less than 0.1 M$_{\textrm{jup}}$ in any of these discs, meaning that giant planets can no longer form in them. Considering that there have been no massive gas discs identified to date in intermediate separation binaries (i.e., binaries at a few hundred au separation), this opens space for speculation whether their binarity causes the removal of gas, with tidal interaction truncating the discs and hence shortening the accretion timescale. More systematic studies in this respect are sorely needed.

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arXiv2020

Nucleosynthetic yields of Z=$10^{-5}$ intermediate-mass stars

Abridged: Observed abundances of extremely metal-poor (EMP) stars in the Halo hold clues for the understanding of the ancient universe. Interpreting these clues requires theoretical stellar models at the low-Z regime. We provide the nucleosynthetic yields of intermediate-mass Z=$10^{-5}$ stars between 3 and 7.5 $M_{sun}$, and quantify the effects of the uncertain wind rates. We expect these yields can be eventually used to assess the contribution to the chemical inventory of the early universe, and to help interpret abundances of selected C-enhanced EMP stars. By comparing our models and other existing in the literature, we explore evolutionary and nucleosynthetic trends with wind prescriptions and with initial metallicity. We compare our results to observations of CEMP-s stars belonging to the Halo. The yields of intermediate-mass EMP stars reflect the effects of very deep second dredge-up (for the most massive models), superimposed with the combined signatures of hot-bottom burning and third dredge-up. We confirm the reported trend that models with initial metallicity Z$_{ini}$ <= 0.001 give positive yields of $^{12}C, ^{15}N, ^{16}O$, and $^{26}Mg$. The $^{20}Ne, ^{21}Ne$, and $^{24}Mg$ yields, which were reported to be negative at Z$_{ini}$ = 0.0001, become positive for Z=$10^{-5}$. The results using two different prescriptions for mass-loss rates differ widely in terms of the duration of the thermally-pulsing (Super) AGB phase, overall efficiency of the third dredge-up episode, and nucleosynthetic yields. The most efficient of the standard wind rates frequently used in the literature seems to favour agreement between our yield results and observational data. Regardless of the wind prescription, all our models become N-enhanced EMP stars.

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arXiv2020

Li-rich K giants, dust excess, and binarity

The origin of the Li-rich K giants is still highly debated. Here, we investigate the incidence of binarity among this family from a nine-year radial-velocity monitoring of a sample of 11 Li-rich K giants using the HERMES spectrograph attached to the 1.2m Mercator telescope. A sample of 13 non-Li-rich giants (8 of them being surrounded by dust according to IRAS, WISE, and ISO data) was monitored alongside. When compared to the binary frequency in a reference sample of 190 K giants (containing 17.4% of definite spectroscopic binaries -- SB -- and 6.3% of possible spectroscopic binaries -- SB?), the binary frequency appears normal among the Li-rich giants (2/11 definite binaries plus 2 possible binaries, or 18.2% SB + 18.2% SB?), after taking account of the small sample size through the hypergeometric probability distribution. Therefore, there appears to be no causal relationship between Li enrichment and binarity. Moreover, there is no correlation between Li enrichment and the presence of circumstellar dust, and the only correlation that could be found between Li enrichment and rapid rotation is that the most Li-enriched K giants appear to be fast-rotating stars. However, among the dusty K giants, the binary frequency is much higher (4/8 definite binaries plus 1 possible binary). The remaining 3 dusty K giants suffer from a radial-velocity jitter, as is expected for the most luminous K giants, which these are.

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arXiv2020

Discovery of technetium- and niobium-rich S stars: the case for bitrinsic stars

S stars are late-type giants with overabundances of s-process elements. They come in two flavours depending on the presence or not of technetium (Tc), an element without stable isotopes. Intrinsic S stars are Tc-rich and genuine asymptotic giant branch (AGB)stars while extrinsic S stars owe their s-process overabundances to the pollution from a former AGB companion, now a white dwarf(WD). In addition to Tc, another distinctive feature between intrinsic and extrinsic S stars is the overabundance of niobium (Nb) in the latter class. We discuss the case of the S stars BD+79 156 and o1Ori whose specificity is to share the distinctive features of both intrinsic and extrinsic S stars, namely the presence of Tc along with a Nb overabundance. BD+79 156 is the first clear case of a bitrinsic star, i.e. a doubly s-process-enriched object, first through mass transfer in a binary system, and then through internal nucleosynthesis (responsible for the Tc-enrichment in BD+79 156 which must therefore have reached the AGB phase of its evolution). This hybrid nature of the s-process pattern in BD+79 156 is supported by its binary nature and its location in the HR diagram just beyond the onset of the third dredge-up on the AGB. The Tc-rich, binary S-star o1Ori with a WD companion was another long-standing candidate for a similar hybrid s-process enrichment. However the marginal overabundance of Nb derived ino1Ori does not allow to trace unambiguously the evidence of a large pollution coming from the AGB progenitor of its current WD companion. As a side product, the current study offers a new way of detecting binary AGB stars with WD companions by identifying their Tc-rich nature along with a Nb overabundance.

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arXiv2019

First grids of low-mass stellar models and isochrones with self-consistent treatment of rotation : From 0.2 to 1.5 M_\odot at 7 metallicities from PMS to TAMS

We present an extended grid of state-of-the art stellar models for low-mass stars including updated physics (nuclear reaction rates, surface boundary condition, mass-loss rate, angular momentum transport, torque and rotation-induced mixing prescriptions). We aim at evaluating the impact of wind braking, realistic atmospheric treatment, rotation and rotation-induced mixing on the structural and rotational evolution from the pre-main sequence to the turn-off. Using the STAREVOL code, we provide an updated PMS grid. We compute stellar models for 7 different metallicities, from [Fe/H] = -1 dex to [Fe/H] = +0.3 dex with a solar composition corresponding to $Z=0.0134$. The initial stellar mass ranges from 0.2 to 1.5\Ms with extra grid refinement around one solar mass. We also provide rotating models for three different initial rotation rates (slow, median and fast) with prescriptions for the wind braking and disc-coupling timescale calibrated on observed properties of young open clusters. The rotational mixing includes an up-to-date description of the turbulence anisotropy in stably stratified regions. The overall behaviour of our models at solar metallicity -- and its constitutive physics -- is validated through a detailed comparison with a variety of distributed evolutionary tracks. The main differences arise from the choice of surface boundary conditions and initial solar composition. The models including rotation with our prescription for angular momentum extraction and self-consistent formalism for angular momentum transport are able to reproduce the rotation period distribution observed in young open clusters over a broad mass-range. These models are publicly available and may be used to analyse data coming from present and forthcoming asteroseismic and spectroscopic surveys such as Gaia, TESS and PLATO.

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arXiv2019

Hertzsprung-Russell diagram and mass distribution of barium stars

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 Msun with a tail at higher masses up to 4.5 Msun. 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 Msun) 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.

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arXiv2019

Barium & related stars and their white-dwarf companions II. Main-sequence and subgiant stars

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 a binary companion when the latter 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 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 inclinations 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 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 orientation 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 interaction along the Red Giant Branch of the Ba star, impacting on the eccentricities and periods of the giants.

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arXiv2018

When binaries keep track of recent nucleosynthesis: The Zr - Nb pair in extrinsic stars as an s-process diagnostic

Barium stars are s-process enriched giants. They owe their chemical peculiarities to a past mass transfer phase. During which they were polluted by their binary companion, which at the time was an 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. 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. Adopting a recently used methodology(Neyskens et al. 2015), we analyse a sample of 18 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, $^{93}Zr$ has fully decayed into mono-isotopic $^{93}Nb$ , so Nb/Zr is a measure of the temperature-sensitive $^{93}Zr/Zr$ isotopic ratio. 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, characterised by high nitrogen abundances, also tend to 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 $^{13}C(alpha,n)^{16}O$ for barium stars. The comparison with stellar evolution and nucleosynthesis models shows that the investigated barium stars were polluted by a low-mass (2-3 Solar mass) AGB star. HD 100503 is potentially identified as the highest metallicity CEMP-rs star yet discovered.

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