7,170 research outputs found
Ages of Exoplanet Host-Stars from Asteroseismology : HD 17156, a Case Study
The characterization of the growing number of newly discovered exoplanets
---nature, internal structure, formation and evolution--- strongly relies on
the properties of their host-star, i.e. its mass, radius and age. These latter
can be inferred from stellar evolution models constrained by the observed
global parameters of the host-star --- effective temperature, photospheric
chemical composition, surface gravity and/or luminosity--- and by its mean
density inferred from the transit analysis. Additional constraints for the
models can be provided by asteroseismic observations of the host-star. The
precision and accuracy on the age, mass and radius not only depend on the
quality and number of available observations of the host-star but also on our
ability to model it properly. Stellar models are still based on a number of
approximations, they rely on physical inputs and data that can be uncertain and
do not treat correctly all the physical processes that can be at work inside a
star. We focus here on the determination of the age of HD 17156, an oscillating
star hosting an exoplanet. We examine the dispersion of the age values obtained
by different methods ---empirical or model-dependent--- and the different
sources of errors ---observational or theoretical--- that intervene in the age
determination based on stellar models.Comment: 8 pages, 5 figures, to appear in the ASP proceedings of "The 61st
Fujihara seminar: Progress in solar/stellar physics with helio- and
asteroseismology", 13th-17th March 2011, Hakone, Japan. Ed: Hiromoto
Shibahash
Impact of asteroseismology on improving stellar ages determination
High precision photometry as performed by the CoRoT and Kepler satellites
on-board instruments has allowed to detect stellar oscillations over the whole
HR diagram. Oscillation frequencies are closely related to stellar interior
properties via the density and sound speed profiles, themselves tightly linked
with the mass and evolutionary state of stars. Seismic diagnostics performed on
stellar internal structure models allow to infer the age and mass of
oscillating stars. The accuracy and precision of the age determination depend
both on the goodness of the observational parameters (seismic and classical)
and on our ability to model a given star properly. They therefore suffer from
any misunderstanding of the physical processes at work inside stars (as
microscopic physics, transport processes...). In this paper, we recall some
seismic diagnostics of stellar age and we illustrate their efficiency in
age-dating the CoRoT target HD 52265.Comment: 10 pages, 2 figures, to be published in the proceedings of the
conference "New advances in stellar physics: from microscopic to macroscopic
processes" held at Roscoff, France. EAS Publications Series, 63 (2013)
123-13
Asteroseismology for "\`{a} la carte" stellar age-dating and weighing: Age and mass of the CoRoT exoplanet host HD 52265
In the context of CoRoT, Kepler, Gaia, TESS, and PLATO, precise and accurate
stellar ages, masses and radii are of paramount importance. They are crucial to
constrain scenarii of planetary formation and evolution.We aim at quantifying
how detailed stellar modeling improves the accuracy and precision on age and
mass of individual stars. We adopt a multifaceted approach where we examine how
the number of observational constraints as well as the uncertainties on
observations and on model input physics impact the age-dating and weighing. We
modelled the exoplanet host-star HD52265, a MS, solar-like oscillator observed
by CoRoT. We considered different sets of observational constraints (HR data,
metallicity, seismic constraints). For each case, we determined the age, mass,
and properties of HD52265 inferred from models, and quantified the impact of
the models inputs. Our seismic analysis provides an age A=2.10-2.54 Gyr, a mass
M=1.14-1.32 Msun, and a radius R=1.30-1.34 Rsun, which corresponds to
uncertainties of 10, 7, and 1.5% respectively. Our seismic study provides
constraints on surface convection, through the mixing-length found to be 12-15%
smaller than the solar one. Because of helium-mass degeneracy, the initial He
abundance is determined modulo the mass. The seismic mass of the exoplanet is
found to be Mp sin i=1.17-1.26 MJup, much more precise than what can be derived
by HR diagram inversion. We demonstrate that asteroseismology allows to improve
the age accuracy compared to other methods. We emphasize that the knowledge of
the mean properties of oscillations -as the large frequency separation- is not
enough for deriving accurate ages. We need precise individual frequencies to
narrow the age scatter due to model uncertainties. This strengthen the case for
precise classical stellar parameters and frequencies as will be obtained by
Gaia and PLATO.Comment: 23 pages, 9 figures, Accepted for publication in Astronomy &
Astrophysics Corrected by the language editor, Table link to CD
Comparisons for Esta-Task3: Cles and Cesam
We present the results of comparing three different implementations of the
microscopic diffusion process in the stellar evolution codes CESAM and CLES.
For each of these implementations we computed models of 1.0, 1.2 and 1.3
M. We analyse the differences in their internal structure at three
selected evolutionary stages, as well as the variations of helium abundance and
depth of the stellar convective envelope. The origin of these differences and
their effects on the seismic properties of the models are also considered.Comment: 10 pages, 8 figures, Joint HELAS and CoRoT/ESTA Workshop on
Solar/Stellar Models and Seismic Analysis Tools, Novembre, Porto 2007 To be
published in EAS Publications Serie
Effects of copper, zinc and selenium status on performance and health in commercial dairy and beef herds: retrospective study
A retrospective study using analysis of plasma copper and zinc, and erythrocyte glutathione peroxidase from 2 080 dairy and beef cow herds was conducted to evaluate the relationship between trace-element status and production, reproduction and health in cows and their calves. Classification of the herd status as deficient, marginal, low-adequate or high-adequate was based on the lower tercile of individual values. Odds ratios for each disorder in herds were calculated by multivariable stepwise logistic regression. Inadequate copper status was not associated with adult disorders, but was an important risk factor for poor calf performance or health. Selenium deficient status was associated with most studied disorders in cows, and both deficient and marginal herd status were strongly associated with poor health of calves, particularly with increased risks of myopathy and infectious diseases. Zinc insufficiency was strongly associated with low milk production and impaired locomotion in dairy herds, and was also associated with diarrhoea and poor growth in calves. Because a low-adequate status increased the risk of many disorders in adults and calves, we propose to classify herds as deficient and marginal when the lower terciles of plasma zinc concentration are below 12 and between 12 and 14 lmol/l respectively
Social Choice with Analytic Preferences
A social welfare function is a mapping from a set of profiles of individual preference orderings to the set of social orderings of a universal set of alternatives. A social choice correspondence specifies a nonempty subset of the agenda for each admissible preference profile and each admissible agenda. We provide examples of economic and political preference domains for which the Arrow social welfare function axioms are inconsistent, but whose choice-theoretic counterparts (with nondictatorship strengthened to anonymity) yield a social choice correspondence possibility theorem when combined with a natural agenda domain. In both examples, agendas are compact subsets of the nonnegative orthant of a multidimensional Euclidean space. In our first possibility theorem, we consider the standard Euclidean spatial model used in many political models. An agenda can be interpreted as being the feasible vectors of public goods given the resource constraints faced by a legislature. Preferences are restricted to be Euclidean spatial preferences. Our second possibility theorem is for economic domains. Alternatives are interpreted as being vectors of public goods. Preferences are monotone and representable by an analytic utility function with no critical points. Convexity of preferences can also be assumed. Many of the utility functions used in economic models, such as Cobb-Douglas and CES, are analytic. Further, the set of monotone, convex, and analytic preference orderings is dense in the set of continuous, monotone, convex preference orderings. Thus, our preference domain is a large subset of the classical domain of economic preferences. An agenda can be interpreted as the set of feasible allocations given an initial resource endowment and the firms' production technologies. To establish this theorem, an ordinal version of the Analytic Continuation Principle is developed.
Stellar convective cores as dark matter probes
The recent detection of a convective core in a main-sequence solar-type star
is used here to test particular models of dark matter (DM) particles, those
with masses and scattering cross sections in the range of interest for the DM
interpretation of the positive results in several DM direct detection
experiments. If DM particles do not effectively self-annihilate after
accumulating inside low-mass stars (e.g. in the asymmetric DM scenario) their
conduction provides an efficient mechanism of energy transport in the stellar
core. For main-sequence stars with masses between 1.1 and 1.3 Msun, this
mechanism may lead to the suppression of the inner convective region expected
to be present in standard stellar evolution theory. The asteroseismic analysis
of the acoustic oscillations of a star can prove the presence/absence of such a
convective core, as it was demonstrated for the first time with the Kepler
field main-sequence solar-like pulsator, KIC 2009505. Studying this star we
found that the asymmetric DM interpretation of the results in the CoGeNT
experiment is incompatible with the confirmed presence of a small convective
core in KIC 2009505.Comment: to appear on Physical Review
Stars in the age of micro-arc-second astrometry
The understanding and modeling of the structure and evolution of stars is
based on statistical physics as well as on hydrodynamics. Today, a precise
identification and proper description of the physical processes at work in
stellar interiors are still lacking (one key point being that of transport
processes) while the comparison of real stars to model predictions, which
implies conversions from the theoretical space to the observational one,
suffers from uncertainties in model atmospheres. That results in uncertainties
on the prediction of stellar properties needed for galactic studies or
cosmology (as stellar ages and masses). In the next decade, progress is
expected from the theoretical, experimental and observational sides. I
illustrate some of the problems we are faced with when modeling stars and the
possible tracks towards their solutions. I discuss how future observational
ground-based or spatial programs (in particular those dedicated to
micro-arc-second astrometry, asteroseismology and interferometry) will provide
precise determinations of the stellar parameters and contribute to a better
knowledge of stellar interiors and atmospheres in a wide range of stellar
masses, chemical compositions and evolution stages.Comment: 7 pages; to appear in the proceedings of "IAU Symposium 248 - A Giant
Step: from Milli- to Micro-arcsecond Astrometry", held in Shanghai, China,
15-19 Oct. 200
Stellar ages from asteroseismology
Asteroseismology provides powerful means to probe stellar interiors. The
oscillations frequencies are closely related to stellar interior properties via
the density and sound speed profiles. Since these are tightly linked with the
mass and evolutionary state, we can expect to determine the age and mass of a
star from the comparison of its oscillation spectrum with predictions of
stellar models. Such a comparison suffers both from the problems we face when
modeling a particular star (as the uncertainties on global parameters and
chemical composition) and from our misunderstanding of processes at work in
stellar interiors (as the transport processes that may lead to core mixing and
affect the model ages). For stars where observations have provided precise and
numerous oscillation frequencies together with accurate global parameters and
additional information (as the radius or the mass if the star is in a binary
system, the interferometric radius or the mean density if the star is an
exoplanet host), we can also expect to better constrain the physical
description of the stellar structure and to get a more reliable age estimation.
After a survey of stellar pulsations, we present some seismic diagnostics that
can be used to infer the age of a star as well as their limitations. We then
illustrate the ability of asteroseismology to scrutinize stellar interiors on
the basis of a few exemples. In the years to come, extended very precise
asteroseismic observations are expected, in photometry or in spectroscopy, from
ground-based (HARPS, CORALIE, ELODIE, UVES, UCLES, SIAMOIS, SONG) or spatial
devices (MOST, CoRoT, WIRE, Kepler, PLATO). This will considerably enlarge the
sample of stars eligible to asteroseismic age determination and should allow to
estimate the age of individual stars with a 10-20% accuracy.Comment: 10 pages, 15 figures, Proc. of the IAU Symp. 258 "The Ages of Stars",
Baltimore USA 13-17 Oct 2008, eds D. Soderblom et al., CUP in pres
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