Barium Stars Research Articles

S stars and s-process in the Gaia era

Context. S stars are transition objects between M-type giants and carbon stars on the asymptotic giant branch (AGB). They are characterized by overabundances of s-process elements. Roughly half of them are enhanced in technetium (Tc), an s-process element with no stable isotope, while the other half lack technetium. This dichotomy arises from the fact that Tc-rich S stars are intrinsically producing s-process elements and have undergone third dredge-up (TDU) events, while Tc-poor S stars owe their s-process overabundances to a past pollution by a former AGB companion which is now an undetected white dwarf, and since the epoch of the mass transfer, technetium has totally decayed. Aims. Our aim is to analyse the abundances of S stars and gain insights into their evolutionary status and on the nucleosynthesis of heavy s-process elements taking place in their interior. In particular, the location of extrinsic and intrinsic S stars in the HR diagram will be compared with the theoretical onset of the TDU on the thermally pulsing AGB. Methods. A sample of 19 S-type stars was analysed by combining HERMES high-resolution spectra, accurate Gaia Data Release 2 (GDR2) parallaxes, stellar-evolution models, and newly designed MARCS model atmospheres for S-type stars. Various stellar parameters impact the atmospheric structure of S stars, not only effective temperature, gravity, metallicity and microturbulence but also C/O and [s/Fe]. We show that photometric data alone are not sufficient to disentangle these parameters. We present a new automatic spectral-fitting method that allows one to constrain the range of possible atmospheric parameters. Results. Combining the derived parameters with GDR2 parallaxes allows a joint analysis of the location of the stars in the Hertzsprung–Russell diagram and of their surface abundances. For all 19 stars, Zr and Nb abundances are derived, complemented by abundances of other s-process elements for the three Tc-rich S stars. These abundances agree within the uncertainties with nucleosynthesis predictions for stars of corresponding mass, metallicity and evolutionary stage. The Tc dichotomy between extrinsic and intrinsic S stars is seen as well in the Nb abundances: intrinsic, Tc-rich S stars are Nb-poor, whereas extrinsic, Tc-poor S stars are Nb-rich. Most extrinsic S stars lie close to the tip of the red giant branch (RGB), and a few are located along the early AGB. All appear to be the cooler analogues of barium stars. Barium stars with masses smaller than 2.5 M⊙ turn into extrinsic S stars on the RGB, because only for those masses does the RGB tip extend to temperatures lower than ~4200 K, which allows the ZrO bands distinctive of S-type stars to develop. On the contrary, barium stars with masses in excess of ~2.5 M⊙ can only turn into extrinsic S stars on the E-AGB, but those are short-lived, and thus rare. The location of intrinsic S stars in the HR diagram is compatible with them being thermally-pulsing AGB stars. Although nucleosynthetic model predictions give a satisfactory distribution of s-process elements, fitting at the same time the carbon and heavy s-element enrichments still remains difficult. Finally, the Tc-rich star V915 Aql is challenging as it points at the occurrence of TDU episodes in stars with masses as low as M ~ 1 M⊙.

The s process in AGB stars as constrained by a large sample of barium stars

Context. Barium (Ba) stars are dwarf and giant stars enriched in elements heavier than iron produced by the slow neutron-capture process (s process). These stars belong to binary systems in which the primary star evolved through the asymptotic giant branch (AGB) phase. During this phase the primary star produced s-process elements and transferred them onto the secondary, which is now observed as a Ba star. Aims. We compare the largest homogeneous set of Ba giant star observations of the s-process elements Y, Zr, La, Ce, and Nd with AGB nucleosynthesis models to reach a better understanding of the s process in AGB stars. Methods. By considering the light-s (ls: Y and Zr) heavy-s (hs: La, Ce, and Nd) and elements individually, we computed for the first time quantitative error bars for the different hs-element to ls-element abundance ratios, and for each of the sample stars. We compared these ratios to low-mass AGB nucleosynthesis models. We excluded La from our analysis because the strong La lines in some of the sample stars cause an overestimation and unreliable abundance determination, as compared to the other observed hs-type elements. Results. All the computed hs-type to ls-type element ratios show a clear trend of increasing with decreasing metallicity with a small spread (less than a factor of 3). This trend is predicted by low-mass AGB models in which 13C is the main neutron source. The comparison with rotating AGB models indicates the need for the presence of an angular momentum transport mechanism that should not transport chemical species, but significantly reduces the rotational speed of the core in the advanced stellar evolutionary stages. This is an independent confirmation of asteroseismology observations of the slow down of core rotation in giant stars, and of rotational velocities of white dwarfs lower than predicted by models without an extra angular momentum transport mechanism.

When binaries keep track of recent nucleosynthesis

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 (CEMP-rs).

When binaries keep track of recent nucleosynthesis

AbstractWe determine Zr and Nb elemental abundances in barium stars to probe the operation temperature of the s-process that occurred in the companion asymptotic giant branch (AGB) stars. Along with Zr and Nb, we derive the abundances of a large number of heavy elements. They provide constraints on the s-process operation temperature and therefore on the s-process neutron source. The results are then compared with stellar evolution and nucleosynthesis models. We compare the nucleosynthetic profile of the present sample stars with those of CEMP-s, CEMP-rs and CEMP-r stars. One barium star of our sample is potentially identified as the highest-metallicity CEMP-rs star yet discovered.

The composition of Barium stars and the s-process in AGB stars

AbstractUsing abundances from the available largest, homogeneous sample of high resolution Barium (Ba) star spectra we calculated the ratios of different hs-like to ls-like elemental ratios and compared to different AGB nucleosynthesis models. The Ba star data show an incontestable increase of the hs-type/ls-type element ratio (for example, [Ce/Y]) with decreasing metallicity. This trend in the Ba star observations is predicted by low mass, non-rotating AGB models where 13C is the main neutron source and is in agreement with Kepler asteroseismology observations.

Binary interaction along the RGB: The Barium Star perspective

AbstractBarium (Ba) stars form via mass-transfer in binary systems, and can subsequently interact with their white dwarf companion in a second stage of binary interaction. We used observations of main-sequence Ba systems as input for our evolutionary models, and try to reproduce the orbits of the Ba giants. We show that to explain short and sometimes eccentric orbits, additional interaction mechanisms are needed along the RGB.

The Interaction Between Accretion from the Interstellar Medium and Accretion from the Evolved Binary Component in Barium Stars

The reanalysis of the previously published abundance pattern of mild barium star HD202109 (ζ Cyg) and the chemicalcompositions of 129 thin disk barium stars facilitated the search for possible correlations of different stellar parameters withsecond ionization potentials of chemical elements. Results show that three valuable correlations exist in the atmospheres ofbarium stars. The first is the relationship between relative abundances and second ionization potentials. The second is theage dependence of mean correlation coefficients of relative abundances vs. second ionization potentials, and the third one isthe changes in correlation coefficients of relative abundances vs. second ionization potentials as a function of stellar spatialvelocities and overabundances of s-process elements. These findings demonstrate the possibility of hydrogen and heliumaccretion from the interstellar medium on the atmospheres of barium stars.

Heavy-element yields and abundances of asymptotic giant branch models with a Small Magellanic Cloud metallicity

We present new theoretical stellar yields and surface abundances for asymptotic giant branch (AGB) models with a metallicity appropriate for stars in the Small Magellanic Cloud (SMC, $Z= 0.0028$, [Fe/H] $\approx -0.7$). New evolutionary sequences and post-processing nucleosynthesis results are presented for initial masses between 1$M_{\odot}$ and 7$M_{\odot}$, where the 7$M_{\odot}$ is a super-AGB star with an O-Ne core. Models above 1.15$M_{\odot}$ become carbon rich during the AGB, and hot bottom burning begins in models $M \ge 3.75 M_{\odot}$. We present stellar surface abundances as a function of thermal pulse number for elements between C to Bi and for a selection of isotopic ratios for elements up to Fe and Ni (e.g., $^{12}$C/$^{13}$C), which can be compared to observations. The integrated stellar yields are presented for each model in the grid for hydrogen, helium and all stable elements from C to Bi. We present evolutionary sequences of intermediate-mass models between 4--7$M_{\odot}$ and nucleosynthesis results for three masses ($M=3.75, 5, 7M_{\odot}$) including $s$-process elements for two widely used AGB mass-loss prescriptions. We discuss our new models in the context of evolved AGB stars and post-AGB stars in the Small Magellanic Clouds, barium stars in our Galaxy, the composition of Galactic globular clusters including Mg isotopes with a similar metallicity to our models, and to pre-solar grains which may have an origin in metal-poor AGB stars.

The Interaction Between Accretion from the Interstellar Medium and Accretion from the Evolved Binary Component in Barium Stars

The reanalysis of the previously published abundance pattern of mild barium star HD202109 (ζ Cyg) and the chemical compositions of 129 thin disk barium stars facilitated the search for possible correlations of different stellar parameters with second ionization potentials of chemical elements. Results show that three valuable correlations exist in the atmospheres of barium stars. The first is the relationship between relative abundances and second ionization potentials. The second is the age dependence of mean correlation coefficients of relative abundances vs. second ionization potentials, and the third one is the changes in correlation coefficients of relative abundances vs. second ionization potentials as a function of stellar spatial velocities and overabundances of s-process elements. These findings demonstrate the possibility of hydrogen and helium accretion from the interstellar medium on the atmospheres of barium stars.

Chemical analysis of three barium stars: HD 51959, HD 88035, and HD 121447

We present elemental abundance results from high resolution spectral analysis of three nitrogen-enhanced barium stars. The analysis is based on spectra obtained with the FEROS attached to 1.52m telescope at ESO, Chile. The spectral resolution is R~48000 and the spectral coverage spans from 3500-9000\AA\,. For the objects HD 51959 and HD 88035, we present the first time abundance analyses results. Although a few studies are available in literature on the object HD 121447, the results are significantly different from each other. We have therefore carried out a detailed chemical composition study for this object based on a high resolution spectrum with high S/N ratio, for a better understanding of the origin of the abundance patterns observed in this star. Stellar atmospheric parameters, the effective temperature, surface gravity, microturbulence and metallicity of the stars are determined from the LTE analysis using model atmospheres. The metallicity of HD 51959 and HD 88035 are found to be near-solar; they exhibit enhanced abundances of neutron-capture elements. HD 121447 is found to be moderately metal-poor with [Fe/H]=-0.65. While carbon is near-solar in the other two objects, HD 121447 shows carbon enhancement at a level, [C/Fe]=0.82. Neutron-capture elements are highly enhanced with [X/Fe]>2 (X: Ba, La, Pr, Nd, Sm) in this object. The alpha- and iron-peak elements show abundances very similar to field giants with the same metallicity. From kinematic analysis all the three objects are found to be members of thin disk population with a high probability of 0.99, 0.99 and 0.92 for HD 51959, HD 88035 and HD 121447 respectively.

Chemical abundances of primary stars in the Sirius-like binary systems

Study of primary stars lying in Sirius-like systems with various masses of WD companions and orbital separations is one of the key aspects to understand the origin and nature of Barium (Ba) stars. In this paper, based on high resolution and high S/N spectra, we present systematic analysis of photospheric abundances for 18 FGK primary stars of Sirius-like systems including six giants and 12 dwarfs. Atmospheric parameters, stellar masses, and abundances of 24 elements (C, Na, Mg, Al, Si, S, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Sr, Y, Zr, Ba, La, Ce and Nd) are determined homogeneously. The abundance patterns in these sample stars show that most of the elements in our sample follow the behavior of field stars with similar metallicity. As expected, s-process elements in four known Ba giants show overabundance. A weak correlation was found between anomalies of s-process elemental abundance and orbital separation, suggesting the orbital separation of the binaries could not be the main constraint to differentiate strong Ba stars from mild Ba stars. Our study shows that the large mass (>0.51 M ) of a WD companion in a binary system is not a sufficient condition to form a Ba star, even if the separation between the two components is small. Although not sufficient it seems to be a necessary condition since Ba stars with lower mass WDs in the observed sample were not found. Our results support that [s/Fe] and [hs/ls] ratios of Ba stars are anti-correlated with the metallicity. However, the different levels of s-process overabundance among Ba stars may not to be dominated mainly by the metallicity.

Carbon Stars Identified from LAMOST DR4 Using Machine Learning

Abstract In this work, we present a catalog of 2651 carbon stars from the fourth Data Release (DR4) of the Large Sky Area Multi-Object Fiber Spectroscopy Telescope (LAMOST). Using an efficient machine-learning algorithm, we find these stars from more than 7 million spectra. As a by-product, 17 carbon-enhanced metal-poor turnoff star candidates are also reported in this paper, and they are preliminarily identified by their atmospheric parameters. Except for 176 stars that could not be given spectral types, we classify the other 2475 carbon stars into five subtypes: 864 C-H, 226 C-R, 400 C-J, 266 C-N, and 719 barium stars based on a series of spectral features. Furthermore, we divide the C-J stars into three subtypes, C-J(H), C-J(R), and C-J(N), and about 90% of them are cool N-type stars as expected from previous literature. Besides spectroscopic classification, we also match these carbon stars to multiple broadband photometries. Using ultraviolet photometry data, we find that 25 carbon stars have FUV detections and that they are likely to be in binary systems with compact white dwarf companions.

Three new barium dwarfs with white dwarf companions: BD+68°1027, RE J0702+129 and BD+80°670

We report three new barium (Ba) dwarfs lying in Sirius-like systems, which provides direct evidence that Ba dwarfs are companions to white dwarfs (WDs). Atmospheric parameters, stellar masses, and chemical abundances of 25 elements, including light, $\alpha$, Fe-peak and s-process elements, are derived from high resolution and high S/N spectra. Enhancement of s-process elements with [s/Fe] ratios between 0.4 and 0.6 confirm them as mild barium stars. The estimated metallicities ($-$0.31, $-$0.06, 0.13) of BD+68$^\circ$1027, RE~J0702+129 and BD+80$^\circ$670 are in the range of known Ba dwarfs and giants. As expected, observed indices of [hs/ls], [s/Fe] and [C/Fe] show anticorrelation with metallicity. AGB progenitor masses are estimated for the WD companions of RE~J0702+129 (1.47 $M_{\odot}$) and BD+80$^\circ$670 (3.59 $M_{\odot}$), which confirms the predicted range of progenitor AGB masses (1.5 $\sim$ 4 $M_{\odot}$) for unseen WDs around Ba dwarfs. Surface abundances of s-process elements in RE~J0702+129 and BD+80$^\circ$670 are compared with AGB models and they are in close agreement, within predicted accretion efficiencies and pollution factors for Ba stars. These results support that the origin of s-process overabundances in Ba dwarfs is similar to Ba giants via McClure hypothesis in which Ba stars accumulate s-process elements through mass transfer from their host companions during AGB phase.

Finding binaries from phase modulation of pulsating stars with Kepler: V. Orbital parameters, with eccentricity and mass-ratio distributions of 341 new binaries

The orbital parameters of binaries at intermediate periods ($10^2$--$10^3$ d) are difficult to measure with conventional methods and are very incomplete. We have undertaken a new survey, applying our pulsation timing method to {\it Kepler} light curves of 2224 main-sequence A/F stars and found 341 non-eclipsing binaries. We calculate the orbital parameters for 317 PB1 systems (single-pulsator binaries) and 24 PB2s (double-pulsators). The method reaches down to small mass ratios $q=0.02$ and yields a highly homogeneous sample. We parametrize the mass-ratio distribution using both inversion and MCMC forward-modelling techniques, and find it to be skewed towards low-mass companions, peaking at $q=0.2$. While solar-type primaries show a brown dwarf desert across intermediate periods, we find a small but significant (2.6$\sigma$) population of extreme-mass-ratio companions ($q<0.1$) to our intermediate-mass primaries. Across periods of 100--1500 d and at $q>0.1$, we measure the binary fraction of current A/F primaries to be 15.4%$\pm$1.4%, though we find that a large fraction of the companions (21%$\pm$6%) are white dwarfs in post-mass-transfer systems with primaries that are now blue stragglers, some of which are the progenitors of Type Ia supernovae, barium stars, symbiotics, and related phenomena. Excluding these white dwarfs, we find the binary fraction of original A/F primaries to be 13.9%$\pm$2.1% over the same parameter space. Combining literature measurements with ours, we find the binary fraction across these periods is a constant 5% for primaries $M_1<0.8$ M$_{\odot}$, but then increases linearly with $\log M_1$, demonstrating that natal discs around more massive protostars $M_1>1$ M$_{\odot}$ become increasingly more prone to fragmentation. Finally, we find the eccentricity distribution of the main-sequence pairs to be much less eccentric than the thermal distribution.

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.

How much mass and angular momentum can the progenitors of carbon-enriched stars accrete?

The chemically peculiar barium stars, CH stars, and most CEMP stars are all believed to be the products of mass transfer in binary systems from a now extinct AGB primary star. The mass of the AGB star and the orbital parameters of the system are the key factors usually considered when determining how much mass is transferred onto the lower-mass main-sequence companion. What is usually neglected, however, is the angular momentum of the accreted material, which should spin up the accreting star. If the star reaches critical rotation, further accretion should cease until the excess angular momentum is somehow dealt with. If the star cannot redistribute or lose the angular momentum while the primary is on the AGB, the amount of mass accreted could be much lower than otherwise expected. Here we present calculations, based on detailed stellar evolution models, of the mass that can be accreted by putative progenitors of Ba and CEMP stars before they reach critical rotation under the assumption that no angular momentum loss occurs during the mass transfer. We consider different accretion rates and values of specific angular momentum. The most stringent limits on the accreted masses result from considering accretion from a Keplerian accretion disk, which is likely present during the formation of most extrinsically-polluted carbon-enriched stars. Our calculations indicate that in this scenario only about 0.05 solar masses of material can be added to the accreting star before it reaches critical rotation, which is much too low to explain the chemical enrichment of many Ba and CEMP stars. Either the specific angular momentum of the accreted material has to effectively be lower by about a factor of ten than the Keplerian value, or significant angular momentum losses must occur for substantial accretion to take place.

The mass-ratio and eccentricity distributions of barium and S stars, and red giants in open clusters

In order to identify diagnostics distinguishing between pre- and post-mass-transfer systems, the mass-ratio distribution and period - eccentricity (P - e) diagram of barium and S stars are compared to those of the sample of binary red giants in open clusters from Mermilliod et al. (2007). From the analysis of the mass-ratio distribution for the cluster binary giants, we find an excess of systems with companion masses between 0.58 and 0.87 Msun, typical for white dwarfs. They represent 22% of the sample, which are thus candidate post-mass-transfer systems. Among these candidates which occupy the same locus as the barium and S stars in the (P-e) diagram, only 33% (= 4/12) show a chemical signature of mass transfer in the form of s-process overabundances (from rather moderate -- about 0.3 dex -- to more extreme -- about 1 dex). These s-process-enriched cluster stars show a clear tendency to be in the clusters with the lowest metallicity in the sample, confirming the classical prediction that the s-process nucleosynthesis is more efficient at low metallicities (the only strong barium star in our sample is found in the cluster with the lowest metallicity, i.e., star 173 in NGC 2420, with [Fe/H] = -0.26). The s-process overabundance is not clearly correlated with the cluster turnoff (TO) mass (such a correlation would instead hint at the importance of the dilution factor). We find as well a mild barium star in NGC 2335, a cluster with a large TO mass of 4.3 Msun, which implies that intermediate-mass AGB stars still operate the s-process and the third dredge-up.

ON THE ECCENTRICITY EXCITATION IN POST-MAIN-SEQUENCE BINARIES

ABSTRACT Several classes of stellar binaries with post-main-sequence (post-MS) components—millisecond pulsars with the white dwarf companions (MSP+WD) and periods of days, binaries hosting post-asymptotic giant branch stars, or barium stars with several years—feature high eccentricities (up to 0.4) despite the expectation of their efficient tidal circularization during their post-MS evolution. It was suggested that the eccentricities of these binaries can be naturally excited by their tidal coupling to the circumbinary disk, formed by the material ejected from the binary. Here we critically reassess this idea using simple arguments rooted in the global angular momentum conservation of the disk+binary system. Compared to previous studies, we (1) fully account for the viscous spreading of the circumbinary disk, (2) consider the possibility of reaccretion from the disk onto the binary (in agreement with simulations and empirical evidence), and (3) allow for the reduced viscosity after the disk expands, cools, and forms dust. These ingredients conspire to significantly lower the efficiency of eccentricity excitation by the disk tides. We find that explaining eccentricities of the post-MS binaries is difficult and requires massive ( ), long-lived ( years) circumbinary disks that do not reaccrete. While disk tides may account for the eccentricities of the MSP+WD binaries, we show reaccretion to also be detrimental for these systems. Reduced efficiency of the disk-driven excitation motivates the study of alternative mechanisms for producing the peculiar eccentricities of the post-MS binaries.

Abundance analysis of s-process enhanced barium stars

Detailed chemical composition studies of stars with enhanced abundances of neutron-capture elements can provide observational constraints for neutron-capture nucleosynthesis studies and clues for understanding their contribution to the Galactic chemical enrichment. We present abundance results from high-resolution spectral analyses of a sample of four chemically peculiar stars characterized by s-process enhancement. High-Resolution spectra (R ~ 42000) of these objects spanning a wavelength range from 4000 to 6800 A, are taken from the ELODIE archive. We have estimated the stellar atmospheric parameters, the effective temperature T_eff, the surface gravity log g, and metallicity [Fe/H] from local thermodynamic equilibrium analysis using model atmospheres. We report estimates of elemental abundances for several neutron-capture elements, Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu and Dy. While HD 49641 and HD 58368 show [Ba/Fe] > 1.16 the other two objects HD 119650 and HD 191010 are found to be mild barium stars with [Ba/Fe] ~ 0.4. The derived abundances of the elements are interpreted on the basis of existing theories for understanding their origin and evolution.

THE BINARY NATURE OF CH-LIKE STARS

ABSTRACT Yamashita has described a group of early carbon stars with enhanced lines of barium that resemble the CH stars but have low radial velocities. It is not clear whether they represent a class of stars separate from early R stars. Radial-velocity measurements and abundance analyses are applied in order to clarify the evolutionary status of CH-like stars. Radial-velocity monitoring was performed over a time interval of about 10 years. Abundance analysis was carried out using high-resolution spectra and the method of atmospheric models for three CH-like candidate stars. The radial-velocity monitoring confirmed regular variations for all of the classified CH-like stars, except for two, in support of their binary nature. The calculated orbital parameters are similar to those observed for barium stars in the disk of the Galaxy and their counterparts in the halo, that is, the CH stars. The relatively low luminosity of CH-like stars and the overabundance of s-process elements in the atmospheres are in agreement with a mass-transfer scenario from the secondary—an AGB star in the past. The kinematic data and metallicities support the idea that CH-like stars are thin/thick-disk population objects.