90 research outputs found
Barium & related stars and their white-dwarf companions I. Giant stars
This paper provides long-period and revised orbits for barium and S stars
adding to previously published ones. The sample of barium stars with strong
anomalies comprise all such stars present in the Lu et al. catalogue. We find
orbital motion for all barium and extrinsic S stars monitored. We obtain the
longest period known so far for a spectroscopic binary involving an S star,
namely 57 Peg with a period of the order of 100 - 500 yr. We present the mass
distribution for the barium stars, which ranges from 1 to 3 Msun, with a tail
extending up to 5 Msun in the case of mild barium stars. This high-mass tail
comprises mostly high-metallicity objects ([Fe/H] >= -0.1). Mass functions are
compatible with WD companions and we derive their mass distribution which
ranges from 0.5 to 1 Msun. Using the initial - final mass relationship
established for field WDs, we derived the distribution of the mass ratio q' =
MAGB,ini / MBa (where MAGB, ini is the WD progenitor initial mass, i.e., the
mass of the system former primary component) which is a proxy for the initial
mass ratio. It appears that the distribution of q' is highly non uniform, and
significantly different for mild and strong barium stars, the latter being
characterized by values mostly in excess of 1.4, whereas mild barium stars
occupy the range 1 - 1.4. We investigate as well the correlation between
abundances, orbital periods, metallicities, and masses (barium star and WD
companion). The 105 orbits of post-mass-transfer systems presented in this
paper pave the way for a comparison with binary-evolution models.Comment: This version 2 is the one accepted by A&A, after language edition.
Paper II about dwarf-Ba and subgiant-CH orbits by Escorza et al. is
arXiv:1904.0409
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.Comment: 14 pages, 17 figure
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
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.Comment: 35 pages, 26 figures, Accepted for publication in A&A, Final versio
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.Comment: Accepted for publication in A&A, 9 pages, 9 figures including the two
in appendi
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.Comment: Accepted for publication in A&A on the 5th of April, 201
A neutron star candidate in the long-period binary 56 UMa
56 UMa is a wide binary system that contains a chemically peculiar red giant
and a faint companion. Due to its surface chemical abundances, the red giant
was classified as a barium (Ba) star. This implies that the companion has to be
a white dwarf, since Ba stars form when mass is transferred to them from an
s-process rich Asymptotic Giant Branch (AGB) star. However, in the case of 56
UMa, the companion might be too massive to be the progeny of an AGB star that
efficiently produced s-process elements such as barium. In this Letter, we
revisit the orbital parameters of the system and perform a full spectral
analysis with the goal of investigating the Ba-star classification of the giant
and unravelling the nature of its faint companion. We combined radial-velocity
and astrometric data to refine the orbital parameters of the system, including
the orbital inclination and the companion mass. Then, we re-determined the
stellar parameters of the giant and its chemical abundances using
high-resolution HERMES spectra. Finally, we investigated the morphology of the
interstellar gas in the vicinity of the system. The faint component in 56 UMa
has a mass of M, which, together with the mixed s+r
abundance profile of the red giant, confirms that the giant is not a standard
barium star. Additionally, the clear identification of a cavity surrounding 56
UMa could indicate that a supernova explosion occurred about 10 5 years ago in
the system, suggesting that the faint companion might be a neutron star.
However, finding an evolutionary scenario that explains all the observables is
not trivial, so we discuss different possible configurations of the system and
their respective merits.Comment: Accepted for publication in A&A
Multi-technique investigation of the binary fraction among A-F type candidate hybrid variable stars discovered by Kepler
Hundreds of candidate hybrid pulsators of intermediate type A-F were revealed by the recent space missions. Hybrid pulsators allow to study the full stellar interiors, where p- and g-modes are simultaneously excited. The true hybrid stars must be identified since other processes, due to stellar multiplicity or rotation, might explain the presence of (some) low frequencies observed in their periodograms. We measured the radial velocities of 50 candidate Delta Sct - Gamma Dor hybrid stars from the Kepler mission with the Hermes/Ace spectrographs over a span of months to years. We aim to derive the fraction of binary and multiple systems and to provide an independent and homogeneous determination of the atmospheric properties and vsini for all targets. The objective is to identify the physical cause of the low frequencies. We computed 1-D cross-correlation functions (CCFs) in order to find the best parameters in terms of the number of components, spectral type and vsini for each target. Radial velocities were measured from spectrum synthesis and by using a 2-D cross-correlation technique in the case of double- and triple-lined systems. Fundamental parameters were determined by fitting (composite) synthetic spectra to the normalised median spectra corrected for the appropriate Doppler shifts. We report on the analysis of 478 high-resolution Hermes and 41 Ace spectra of A/F-type candidate hybrid pulsators from the Kepler field. We determined their radial velocities, projected rotational velocities, atmospheric properties and classified our targets based on the shape of the CCFs and the temporal behaviour of the radial velocities. We derived orbital solutions for seven new systems. Three long-period preliminary orbital solutions are confirmed by a photometric time-delay analysis. Finally, we determined a global multiplicity fraction of 27% in our sample of candidate hybrid stars
Cannibals in the thick disk: The young α-rich stars as evolved blue stragglers
Spectro-seismic measurements of red giants enabled the recent discovery of stars in the thick disk that are more massive than 1.4 . While it has been claimed that most of these stars are younger than the rest of the typical thick disk stars, we show evidence that they might be products of mass transfer in binary evolution, notably evolved blue stragglers. We took new measurements of the radial velocities in a sample of 26 stars from APOKASC, including 13 âyoungâ stars and 13 âoldâ stars with similar stellar parameters but with masses below 1.2 and found that more of the young starsappear to be in binary systems with respect to the old stars.Furthermore, we show that the young stars do not follow the expected trend of [C/H] ratios versus mass for individual stars. However, with a population synthesis of low-mass stars including binary evolution and mass transfer, we can reproduce the observed [C/N] ratios versus mass. Our study shows how asteroseismology of solar-type red giants provides us with a unique opportunity to study the evolution of field blue stragglers after they have left the main-sequence.This work was partly supported by the European Union FP7 programme through ERC grant number 320360. P.J. acknowledges Kingâs College Cambridge for partially supporting this work. K.H. is supported by Marshall Scholarship and Kingâs College Cambridge Studenship. R.J.I. thanks the STFC for funding his Rutherford Fellowship. Based on observations made with the Mercator Telescope, operated on the island of La Palma by the Flemish Community, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de AstrofĂ sica de Canarias. Based on observations obtained with the HERMES spectrograph, which is supported by the Research Foundation â Flanders (FWO), Belgium, the Research Council of KU Leuven, Belgium, the Fonds National de la Recherche Scientifique (F.R.S. â FNRS), Belgium, the Royal Observatory of Belgium, the Observatoire de GenĂšve, Switzerland and the ThĂŒringer Landessternwarte Tautenburg, Germany.This is the final version of the article. It first appeared from EDP Sciences via https://doi.org/10.1051/0004-6361/20162935
Site fidelity and movement patterns of short-finned pilot whales within the Canary Islands : evidence for resident and transient populations
Funding: co-funded by the Canary Government (ConsejerĂa de PolĂtica Territorial, Sostenibilidad y Seguridad), the Spanish Government (FundaciĂłn Biodiversidad and Ministerio de Medio Ambiente, Medio Rural y Marino), FundaciĂłn La Caixa, and by a number of international projects funded by EU programmes MACETUS (FEDER/INTERREG III-B MAC/4.2/M10), EMECETUS (FEDER/INTERREG III-B56105/MAC/4.2/M10), LIFE (LIFE03NAT0062), INDEMARES LIFE+ (LIFE07/NAT/E/00732).1. The geographic location and oceanographic, physical, and chemical water properties make the Canary Islands one of the planet's biodiversity hotspots. The shortâfinned pilot whale (Globicephala macrorhynchus) is one of the archipelago's most commonly encountered species and is potentially vulnerable to a range of anthropogenic pressures, including habitat degradation, acoustic pollution, fishing, whaleâwatching operations, and shipping. Assessment of impact has not been possible because of a lack of even basic information about occurrence and distribution. 2. Spatial and temporal distributions, ranging behaviour, and residence patterns of shortâfinned pilot whales were explored for the first time using survey and photoâidentification data collected in the Canary Islands between 1999 and 2012. In total, 1,081 pilot whale sightings were recorded during 70,620 km of search effort over 1,782 survey days. 3. Pilot whales were detected year round and distributed nonâuniformly within the archipelago, with greater densities concentrated in patchy areas mainly on the leeward side of the main islands. In total, 1,320 wellâmarked individuals were identified, which exhibited a large degree of variability in site fidelity. 4. Different but not isolated subpopulations of pilot whales that share ranges and maintain social interactions were apparently present in the Canary Islands. Strong evidence of an islandâassociated subpopulation was found, with a group of 50 âcore residentâ individuals associated particularly with Tenerife. There were also âtransientâ individuals or temporary migrants, which, probably driven by interâ and intraâspecific competition, may travel long distances whilst using the archipelago as part of a larger range. 5. These findings fill a major gap in the knowledge of this speciesâ occurrence, distribution, movements, and site fidelity in the archipelago and provide much needed data to allow the initiation of informed conservation assessments and management actions.PostprintPeer reviewe
- âŠ