255 research outputs found
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, 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 K, , [Fe/H] = 0.09
and 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. 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 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
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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
. So why do the masses of a sizeable fraction of
thick-disc stars exceed , with some as massive as
? 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 . Of these stars, most are single
because they are merged binaries. Some stars more massive than
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
. 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
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