2,082 research outputs found
Constraining angular momentum transport processes in stellar interiors with red-giant stars in the open cluster NGC6819
Clusters are excellent test benches for verification and improvement of
stellar evolution theory. The recent detection of solar-like oscillations in
G-K giants in the open cluster NGC6819 with Kepler provides us with independent
constraints on the masses and radii of stars on the red giant branch, as well
as on the distance to clusters and their ages. We present, for NGC6819,
evolutionary models by considering rotation-induced mixing ; and the
theoretical low-l frequencies of our stellar models.Comment: Submitted to EPJ Web of Conferences, to appear in the Proceedings of
the 3rd CoRoT Symposium, Kepler KASC7 joint meeting; 2 pages, 1 figur
Thermohaline instability and rotation-induced mixing. III - Grid of stellar models and asymptotic asteroseismic quantities from the pre-main sequence up to the AGB for low- and intermediate-mass stars at various metallicities
The availability of asteroseismic constraints for a large sample of stars
from the missions CoRoT and Kepler paves the way for various statistical
studies of the seismic properties of stellar populations. In this paper, we
evaluate the impact of rotation-induced mixing and thermohaline instability on
the global asteroseismic parameters at different stages of the stellar
evolution from the Zero Age Main Sequence to the Thermally Pulsating Asymptotic
Giant Branch to distinguish stellar populations. We present a grid of stellar
evolutionary models for four metallicities (Z = 0.0001, 0.002, 0.004, and
0.014) in the mass range between 0.85 to 6.0 Msun. The models are computed
either with standard prescriptions or including both thermohaline convection
and rotation-induced mixing. For the whole grid we provide the usual stellar
parameters (luminosity, effective temperature, lifetimes, ...), together with
the global seismic parameters, i.e. the large frequency separation and
asymptotic relations, the frequency corresponding to the maximum oscillation
power {\nu}_{max}, the maximal amplitude A_{max}, the asymptotic period spacing
of g-modes, and different acoustic radii. We discuss the signature of
rotation-induced mixing on the global asteroseismic quantities, that can be
detected observationally. Thermohaline mixing whose effects can be identified
by spectroscopic studies cannot be caracterized with the global seismic
parameters studied here. But it is not excluded that individual mode
frequencies or other well chosen asteroseismic quantities might help
constraining this mixing.Comment: 15 pages, 11 figures, accepted for publication in A&
Beryllium abundances along the evolutionary sequence of the open cluster IC 4651 - New test for hydrodynamical stellar models
[abridged] Previous analyses of lithium abundances in main sequence and red
giant stars have revealed the action of mixing mechanisms other than convection
in stellar interiors. Beryllium abundances in stars with lithium abundance
determinations can offer valuable complementary information on the nature of
these mechanisms. Our aim is to derive beryllium abundances along the whole
evolutionary sequence of an open cluster, IC 4651. These Be abundances are used
together with previously determined Li abundances, in the same sample stars, to
investigate the mixing mechanisms in a range of stellar masses and evolutionary
stages. New beryllium abundances are determined from high-resolution, high
signal-to-noise UVES spectra using spectrum synthesis and model atmospheres.
The careful synthetic modelling of the Be lines region is used to calculate
reliable abundances in rapidly rotating stars. The observed behavior of Be and
Li is compared to theoretical predictions from stellar models including
rotation-induced mixing, internal gravity waves, atomic diffusion, and
thermohaline mixing. Beryllium is detected in all the main sequence and
turn-off sample stars, both slow- and fast-rotating stars, including the Li-dip
stars, but was not detected in the red giants. Confirming previous results, we
find that the Li dip is also a Be dip, although the depletion of Be is more
modest than that of Li in the corresponding effective temperature range. For
post-main-sequence stars, the Be dilution starts earlier within the Hertzsprung
gap than expected from classical predictions as does the Li dilution. A clear
dispersion in the Be abundances is also observed. Theoretical stellar models
including the hydrodynamical transport processes mentioned above are able to
reproduce well all the observed features.Comment: 12 pages, accepted for publication in A&A, revised final versio
Beryllium abundances along the evolutionary sequence of the open cluster IC 4651
The simultaneous investigation of Li and Be in stars is a powerful tool in the study of the evolutionary mixing processes. Here, we present beryllium abundances in stars along the whole evolutionary sequence of the open cluster IC 4651. This cluster has a metallicity of [Fe/H] = +0.11 and an age of 1.2 or 1.7 Gyr. Abundances have been determined from high-resolution, high signal-to-noise UVES spectra using spectrum synthesis and model atmospheres. Lithium abundances for the same stars were determined in a previous work. Confirming previous results, we find that the Li dip is also a Be dip. For post-main-sequence stars, the Be dilution starts earlier within the Hertzsprung gap than expected from classical predictions, as does the Li dilution. Theoretical hydrodynamical models are able to reproduce well all the observed feature
Challenges of payment-for-performance in health care and other public services – design, implementation and evaluation
Interest in mechanisms linking varying proportions of provider payments to achieving certain performance targets has recently grown in health care and other public services in an attempt to solve the cost-quality conundrum. Against the backdrop of this enthusiasm, it is important to review the challenges posed in the process of designing, implementing and evaluating of pay-for-performance
(P4P) in health care and in other UK public services
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)
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)
Study of small distorsions in Bi12MO20 oxides by simple exafs transmission technics
International audienceIn some Bi12 [A+xα B+yβ]O20 sillenite type phases the interatomic distances between oxygen and A or B cations located in tetrahedral sites of the lattice can be determined by EXAFS despite difficult recording conditions due to relatively low concentrations of A and B species in presence of strong absorbing bismuth
Atmospheric parameters and chemical properties of red giants in the CoRoT asteroseismology fields
A precise characterisation of the red giants in the seismology fields of the
CoRoT satellite is a prerequisite for further in-depth seismic modelling.
High-resolution FEROS and HARPS spectra were obtained as part of the
ground-based follow-up campaigns for 19 targets holding great asteroseismic
potential. These data are used to accurately estimate their fundamental
parameters and the abundances of 16 chemical species in a self-consistent
manner. Some powerful probes of mixing are investigated (the Li and CNO
abundances, as well as the carbon isotopic ratio in a few cases). The
information provided by the spectroscopic and seismic data is combined to
provide more accurate physical parameters and abundances. The stars in our
sample follow the general abundance trends as a function of the metallicity
observed in stars of the Galactic disk. After an allowance is made for the
chemical evolution of the interstellar medium, the observational signature of
internal mixing phenomena is revealed through the detection at the stellar
surface of the products of the CN cycle. A contamination by NeNa-cycled
material in the most massive stars is also discussed. With the asteroseismic
constraints, these data will pave the way for a detailed theoretical
investigation of the physical processes responsible for the transport of
chemical elements in evolved, low- and intermediate-mass stars.Comment: Accepted for publication in A&A, 25 pages, 13 colour figures (revised
version after language editing
Dark-bright mixing of interband transitions in symmetric semiconductor quantum dots
In photoluminescence spectra of symmetric [111] grown GaAs/AlGaAs quantum
dots in longitudinal magnetic fields applied along the growth axis we observe
in addition to the expected bright states also nominally dark transitions for
both charged and neutral excitons. We uncover a strongly non-monotonous, sign
changing field dependence of the bright neutral exciton splitting resulting
from the interplay between exchange and Zeeman effects. Our theory shows
quantitatively that these surprising experimental results are due to
magnetic-field-induced \pm 3/2 heavy-hole mixing, an inherent property of
systems with C_3v point-group symmetry.Comment: 5 pages, 3 figure
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