87 research outputs found
The AGB bump: a calibrator for the core mixing
The efficiency of convection in stars affects many aspects of their evolution
and remains one of the key-open questions in stellar modelling. In particular,
the size of the mixed core in core-He-burning low-mass stars is still uncertain
and impacts the lifetime of this evolutionary phase and, e.g., the C/O profile
in white dwarfs. One of the known observables related to the Horizontal Branch
(HB) and Asymptotic Giant Branch (AGB) evolution is the AGB bump. Its
luminosity depends on the position in mass of the helium-burning shell at its
first ignition, that is affected by the extension of the central mixed region.
In this preliminary work we show how various assumptions on near-core mixing
and on the thermal stratification in the overshooting region affect the
luminosity of the AGB bump, as well as the period spacing of gravity modes in
core-He-burning models.Comment: Submitted to EPJ Web of Conferences, to appear in the Proceedings of
the 3rd CoRoT Symposium, Kepler KASC7 joint meeting; 2 pages, 2 figure
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
Grids of Stellar Models and Frequencies with CLES + LOSC
We present a grid of stellar models, obtained with the CLES evolution code,
following the specification of ESTA-Task1, and the corresponfing seismic
properties, computed with the LOSC code. We provide a complete description of
the corresponding files that will be available on the ESTA web-pages.Comment: 8 pages, accepted for publication in Astrophys. Space Sci.
(CoRoT/ESTA Volume
Investigating surface correction relations for RGB stars
State-of-the-art stellar structure and evolution codes fail to adequately
describe turbulent convection. For stars with convective envelopes, such as red
giants, this leads to an incomplete depiction of the surface layers. As a
result, the predicted stellar oscillation frequencies are haunted by systematic
errors, the so-called surface effect. Different empirically and theoretically
motivated correction relations have been proposed to deal with this issue. In
this paper, we compare the performance of these surface correction relations
for red giant branch stars. For this purpose, we apply the different surface
correction relations in asteroseismic analyses of eclipsing binaries and open
clusters. In accordance with previous studies of main-sequence stars, we find
that the use of different surface correction relations biases the derived
global stellar properties, including stellar age, mass, and distance estimates.
We furthermore demonstrate that the different relations lead to the same
systematic errors for two different open clusters. Our results overall
discourage from the use of surface correction relations that rely on reference
stars to calibrate free parameters. Due to the demonstrated systematic biasing
of the results, the use of appropriate surface correction relations is
imperative to any asteroseismic analysis of red giants. Accurate mass, age, and
distance estimates for red giants are fundamental when addressing questions
that deal with the chemo-dynamical evolution of the Milky Way galaxy. In this
way, our results also have implications for fields, such as galactic
archaeology, that draw on findings from stellar physics
The Red-Giant Branch Bump Revisited::Constraints on Envelope Overshooting in a Wide Range of Masses and Metallicities
The red-giant branch bump provides valuable information for the investigation
of the internal structure of low-mass stars. Because current models are unable
to accurately predict the occurrence and efficiency of mixing processes beyond
convective boundaries, one can use the luminosity of the bump --- a diagnostic
of the maximum extension of the convective envelope during the first-dredge up
--- as a calibrator for such processes. By combining asteroseismic and
spectroscopic constraints, we expand the analysis of the bump to masses and
metallicities beyond those previously accessible using globular clusters. Our
dataset comprises nearly 3000 red-giant stars observed by {\it Kepler} and with
APOGEE spectra. Using statistical mixture models, we are able to detect the
bump in the average seismic parameters and , and show that its observed position reveals general trends with mass
and metallicity in line with expectations from models. Moreover, our analysis
indicates that standard stellar models underestimate the depth of efficiently
mixed envelopes. The inclusion of significant overshooting from the base of the
convective envelope, with an efficiency that increases with decreasing
metallicity, allows to reproduce the observed location of the bump.
Interestingly, this trend was also reported in previous studies of globular
clusters.Comment: 9 pages, 5 figures, Accepted for publication in Ap
Probing the properties of convective cores through g modes: high-order g modes in SPB and gamma Doradus stars
In main sequence stars the periods of high-order gravity modes are sensitive
probes of stellar cores and, in particular, of the chemical composition
gradient that develops near the outer edge of the convective core. We present
an analytical approximation of high-order g modes that takes into account the
effect of the mu gradient near the core. We show that in main-sequence models,
similarly to the case of white dwarfs, the periods of high-order gravity modes
are accurately described by a uniform period spacing superposed to an
oscillatory component. The periodicity and amplitude of such component are
related, respectively, to the location and sharpness of the mu gradient.
We investigate the properties of high-order gravity modes for stellar models
in a mass domain between 1 and 10 Msun, and the effects of the stellar mass,
evolutionary state, and extra-mixing processes on period spacing features. In
particular, we show that for models of a typical SPB star, a chemical mixing
that could likely be induced by the slow rotation observed in these stars, is
able to significantly change the g-mode spectra of the equilibrium model.
Prospects and challenges for the asteroseismology of gamma Doradus and SPB
stars are also discussed.Comment: 18 pages, 29 figures, accepted for publication in MNRA
Thorough analysis of input physics in CESAM and CLES codes
This contribution is not about the quality of the agreement between stellar
models computed by CESAM and CLES codes, but more interesting, on what
ESTA-Task~1 run has taught us about these codes and about the input physics
they use. We also quantify the effects of different implementations of the same
physics on the seismic properties of the stellar models, that in fact is the
main aim of ESTA experiments.Comment: 11 pages, 12 fig. Accepted for publication in ApSS CoRoT/ESTA Volu
On the impact of the structural surface effect on global stellar properties and asteroseismic analyses
In a series of papers, we have recently demonstrated that it is possible to
construct stellar structure models that robustly mimic the stratification of
multi-dimensional radiative magneto-hydrodynamic simulations at every time-step
of the computed evolution. The resulting models offer a more realistic
depiction of the near-surface layers of stars with convective envelopes than
parameterizations, such as mixing length theory, do. In this paper, we explore
how this model improvement impacts on seismic and non-seismic properties of
stellar models across the Hertzsprung-Russell diagram. We show that the
improved description of the outer boundary layers alters the predicted global
stellar properties at different evolutionary stages. In a hare and hound
exercise, we show that this plays a key role for asteroseismic analyses, as it,
for instance, often shifts the inferred stellar age estimates by more than 10
per cent. Improper boundary conditions may thus introduce systematic errors
that exceed the required accuracy of the PLATO space mission. Moreover, we
discuss different approximations for how to compute stellar oscillation
frequencies. We demonstrate that the so-called gas approximation
performs reasonably well for all main-sequence stars. Using a Monte Carlo
approach, we show that the model frequencies of our hybrid solar models are
consistent with observations within the uncertainties of the global solar
parameters when using the so-called reduced approximation.Comment: Submitted to MNRA
Stellar Spin-Orbit Misalignment in a Multiplanet System
Stars hosting hot Jupiters are often observed to have high obliquities,
whereas stars with multiple co-planar planets have been seen to have low
obliquities. This has been interpreted as evidence that hot-Jupiter formation
is linked to dynamical disruption, as opposed to planet migration through a
protoplanetary disk. We used asteroseismology to measure a large obliquity for
Kepler-56, a red giant star hosting two transiting co-planar planets. These
observations show that spin-orbit misalignments are not confined to hot-Jupiter
systems. Misalignments in a broader class of systems had been predicted as a
consequence of torques from wide-orbiting companions, and indeed
radial-velocity measurements revealed a third companion in a wide orbit in the
Kepler-56 system.Comment: Accepted for publication in Science, published online on October 17
2013; PDF includes main article and supplementary materials (65 pages, 27
figures, 7 tables); v2: small correction to author lis
- …