IP Eri: A surprising long-period binary system hosting a He white dwarf

We determine the orbital elements for the K0 IV + white dwarf (WD) system IP Eri, which appears to have a surprisingly long period of 1071 d and a significant eccentricity of 0.25. Previous spectroscopic analyses of the WD, based on a distance of 101 pc inferred from its Hipparcos parallax, yielded a mass of only 0.43 M$_\odot$, implying it to be a helium-core WD. The orbital properties of IP Eri are similar to those of the newly discovered long-period subdwarf B star (sdB) binaries, which involve stars with He-burning cores surrounded by extremely thin H envelopes, and are therefore close relatives to He WDs. We performed a spectroscopic analysis of high-resolution spectra from the HERMES/Mercator spectrograph and concluded that the atmospheric parameters of the K0 component are $T_{\rm eff} = 4960$ K, $\log{g} = 3.3$, [Fe/H] = 0.09 and $\xi = 1.5$ km/s. The detailed abundance analysis focuses on C, N, O abundances, carbon isotopic ratio, light (Na, Mg, Al, Si, Ca, Ti) and s-process (Sr, Y, Zr, Ba, La, Ce, Nd) elements. We conclude that IP Eri abundances agree with those of normal field stars of the same metallicity. The long period and non-null eccentricity indicate that this system cannot be the end product of a common-envelope phase; it calls instead for another less catastrophic binary-evolution channel presented in detail in a companion paper (Siess et al. 2014).Comment: 14 pages, 10 figures, 4 tables, accepted for publication in A&A (Update of Table 3, Fig. 8 and text in Sect. 5.1, 5.3 and 6 due to minor corrections on N and Y II

New determination of abundances and stellar parameters for a set of weak G-band stars

Weak G-band (wGb) stars are very peculiar red giants almost devoided of carbon and often mildly enriched in lithium. Despite their very puzzling abundance patterns, very few detailed spectroscopic studies existed up to a few years ago, preventing any clear understanding of the wGb phenomenon. We recently proposed the first consistent analysis of published data for 28 wGb stars and identified them as descendants of early A-type to late B-type stars, without being able to conclude on their evolutionary status or the origin of their peculiar abundance pattern. We used newly obtained high-resolution and high SNR spectra for 19 wGb stars in the southern and northern hemisphere to homogeneously derive their fundamental parameters, metallicities, as well as the spectroscopic abundances for Li, C, N, O, Na, Sr, and Ba. We also computed dedicated stellar evolution models that we used to determine the masses and to investigate the evolutionary status and chemical history of the stars in our sample. We confirm that the wGb stars are stars in the mass range 3.2 to 4.2 M$_\odot$. We suggest that a large fraction could be mildly evolved stars on the SGB currently undergoing the 1st DUP, while a smaller number of stars are more probably in the core He burning phase at the clump. After analysing their abundance pattern, we confirm their strong N enrichment anti-correlated with large C depletion, characteristic of material fully processed through the CNO cycle to an extent not known in other evolved intermediate-mass stars. However, we demonstrate here that such a pattern is very unlikely due to self-enrichment. In the light of the current observational constraints, no solid self-consistent pollution scenario can be presented either, leaving the wGb puzzle largely unsolved.Comment: 19 pages , 14 figures, accepted for publication in Astronomy & Astrophysic

A holistic approach to carbon-enhanced metal-poor stars

By considering the various CEMP subclasses separately, we try to derive, from the specific signatures imprinted on the abundances, parameters (such as metallicity, mass, temperature, and neutron source) characterizing AGB nucleosynthesis from the specific signatures imprinted on the abundances, and separate them from the impact of thermohaline mixing, first dredge-up, and dilution associated with the mass transfer from the companion.To put CEMP stars in a broad context, we collect abundances for about 180 stars of various metallicities, luminosity classes, and abundance patterns, from our own sample and from literature. First, we show that there are CEMP stars which share the properties of CEMP-s stars and CEMP-no stars (which we call CEMP-low-s stars). We also show that there is a strong correlation between Ba and C abundances in the s-only CEMP stars. This strongly points at the operation of the 13C neutron source in low-mass AGB stars. For the CEMP-rs stars (seemingly enriched with elements from both the s- and r-processes), the correlation of the N abundances with abundances of heavy elements from the 2nd and 3rd s-process peaks bears instead the signature of the 22Ne neutron source. Adding the fact that CEMP-rs stars exhibit O and Mg enhancements, we conclude that extremely hot conditions prevailed during the thermal pulses of the contaminating AGB stars. Finally, we argue that most CEMP-no stars (with no overabundances for the neutron-capture elements) are likely the extremely metal-poor counterparts of CEMP neutron-capture-rich stars. We also show that the C enhancement in CEMP-no stars declines with metallicity at extremely low metallicity ([Fe/H]~< -3.2). This trend is not predicted by any of the current AGB models.Comment: 27 pages, 24 figures, accepted for publication in A&

Nitrogen depletion in field red giants: mixing during the He flash?

We combine simultaneous constraints on stellar evolutionary status from asteroseismology, and on nitrogen abundances derived from large spectroscopic surveys, to follow nitrogen surface abundances all along the evolution of a low-mass star, comparing model expectations with data. After testing and calibrating the observed yields from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey, we first show that nitrogen surface abundances follow the expected trend after the first dredge-up occurred, i.e. that the more massive is the star, the more nitrogen is enhanced. Moreover, the behaviour of nitrogen data along the evolution confirms the existence of non-canonical extramixing on the red giant branch (RGB) for all low-mass stars in the field. But more surprisingly, the data indicate that nitrogen has been depleted between the RGB tip and the red clump. This may suggest that some nitrogen has been burnt near or at the He flash episode.This work was partly supported by the European Union FP7 programme through ERC grant number 320360. NL acknowledges financial support from the Marie Curie Intra-European fellowship (FP7-PEOPLE-2012-IEF) and the CNES postdoctoral fellowship 2016. AM and YE acknowledge the support of the UK Science and Technology Facilities Council (STFC). Funding for the Stellar Astrophysics Centre (SAC) is provided by The Danish National Research Foundation (Grant agreement no. DNRF106)

CH in stellar atmospheres: an extensive linelist

The advent of high-resolution spectrographs and detailed stellar atmosphere modelling has strengthened the need for accurate molecular data. Carbon-enhanced metal-poor (CEMP) stars spectra are interesting objects with which to study transitions from the CH molecule. We combine programs for spectral analysis of molecules and stellar-radiative transfer codes to build an extensive CH linelist, including predissociation broadening as well as newly identified levels. We show examples of strong predissociation CH lines in CEMP stars, and we stress the important role played by the CH features in the Bond-Neff feature depressing the spectra of barium stars by as much as 0.2 magnitudes in the $\lambda=$3000 -- 5500 \AA\ range. Because of the extreme thermodynamic conditions prevailing in stellar atmospheres (compared to the laboratory), molecular transitions with high energy levels can be observed. Stellar spectra can thus be used to constrain and improve molecular data.Comment: 33pages, 15 figures, accepted in A&A external data available at http://www.astro.ulb.ac.be/~spectrotools

Binary properties of CH and Carbon-Enhanced Metal-Poor stars

The HERMES spectrograph installed on the 1.2-m Mercator telescope has been used to monitor the radial velocity of 13 low-metallicity carbon stars, among which 7 Carbon-Enhanced Metal-Poor (CEMP) stars and 6 CH stars. All stars but one show clear evidence for binarity. New orbits are obtained for 8 systems. The sample covers an extended range in orbital periods, extending from 3.4 d (for the dwarf carbon star HE 0024-2523) to about 54 yr (for the CH star HD 26, the longest known among barium, CH and extrinsic S stars). Three systems exhibit low-amplitude velocity variations with periods close to 1 yr superimposed on a long-term trend. In the absence of an accurate photometric monitoring of these systems, it is not clear yet whether these variations are the signature of a very low-mass companion, or of regular envelope pulsations. The period - eccentricity (P - e) diagram for the 40 low-metallicity carbon stars with orbits now available shows no difference between CH and CEMP-s stars (the latter corresponding to those CEMP stars enriched in s-process elements, as are CH stars). We suggest that they must be considered as one and the same family and that their different names only stem from historical reasons. Indeed, these two families have as well very similar mass-function distributions, corresponding to companions with masses in the range 0.5 - 0.7 Msun, indicative of white-dwarf companions, adopting 0.8 - 0.9 Msun for the primary component. This result confirms that CH and CEMP-s stars obey the same mass-transfer scenario as their higher-metallicity analogs, the barium stars. The P - e diagrams of barium, CH and CEMP-s stars are indeed very similar. They reveal two different groups of systems: one with short orbital periods (P < 1000 d) and mostly circular or almost circular orbits, and another with longer-period and eccentric (e > 0.1) orbits.Comment: Accepted in Astronomy & Astrophysic

Binary stars in the Galactic thick disc

The combination of asteroseismologically-measured masses with abundances from detailed analyses of stellar atmospheres challenges our fundamental knowledge of stars and our ability to model them. Ancient red-giant stars in the Galactic thick disc are proving to be most troublesome in this regard. They are older than 5 Gyr, a lifetime corresponding to an initial stellar mass of about $1.2{\mathrm{M}_{\odot}}$. So why do the masses of a sizeable fraction of thick-disc stars exceed $1.3{\mathrm{M}_{\odot}}$, with some as massive as $2.3{\mathrm{M}_{\odot}}$ ? We answer this question by considering duplicity in the thick-disc stellar population using a binary population-nucleosynthesis model. We examine how mass transfer and merging affect the stellar mass distribution and surface abundances of carbon and nitrogen. We show that a few per cent of thick-disc stars can interact in binary star systems and become more massive than $1.3{\mathrm{M}_{\odot}}$. Of these stars, most are single because they are merged binaries. Some stars more massive than $1.3{\mathrm{M}_{\odot}}$ form in binaries by wind mass transfer. We compare our results to a sample of the APOKASC data set and find reasonable agreement except in the number of these thick-disc stars more massive than $1.3{\mathrm{M}_{\odot}}$. This problem is resolved by the use of a logarithmically-flat orbital-period distribution and a large binary fraction

A VLT-UVES spectrscopic analysis of C-rich Fe-poor stars

Large surveys of very metal-poor stars have revealed in recent years that a large fraction of these objects were carbon-rich, analogous to the more metal-rich CH-stars. The abundance peculiarities of CH-stars are commonly explained by mass-transfer from a more evolved companion. In an effort to better understand the origin and importance for Galactic evolution of Fe-poor, C-rich stars, we present abundances determined from high-resolution and high signal-to-noise spectra obtained with the UVES instrument attached to the ESO/VLT. Our analysis of carbon-enhanced objects includes both CH stars and more metal-poor objects, and we explore the link between the two classes. We also present preliminary results of our ongoing radial velocity monitoring.Comment: 3 pages, 4 figures, presented at Cool Stars 13, Hamburg, 200