1,048 research outputs found
Testing the cores of first ascent red-giant stars using the period spacing of g modes
In the context of the determination of stellar properties using
asteroseismology, we study the influence of rotation and convective-core
overshooting on the properties of red-giant stars. We used models in order to
investigate the effects of these mechanisms on the asymptotic period spacing of
gravity modes () of red-giant stars that ignite He burning in
degenerate conditions (M2.0 M). We also compare the
predictions of these models with Kepler observations. For a given ,
depends not only on the stellar mass, but also on mixing
processes that can affect the structure of the core. We find that in the case
of more evolved red-giant-branch (RGB) stars and regardless of the transport
processes occurring in their interiors, the observed can provide
information as to their stellar luminosity, within ~10-20%. In general, the
trends of with respect to mass and metallicity that are observed
in Kepler red-giant stars are well reproduced by the models.Comment: 5pages, 6 figure
Asteroseismology of red giant stars: a tool for constraining stellar models
The aim of this thesis is to study stellar evolution and asteroseimology of red-giant stars mainly from a modelling point of view, in particular the impact on core-convective-burning stars of adopting different mixing schemes.
Thanks to NASA space telescope Kepler, asteroseismology of thousands of giants provided us new information related to their internal structure, that can be used for finding constraints on their cores.
I used several stellar evolution codes (MESA, BaSTI, and PARSEC) to investigate the effect of different mixing schemes in the helium-core-burning stars. Comparing them with observed stars, I concluded that standard stellar models, largely used in literature, cannot describe the combined observed distribution of luminosity and period spacing. I then proposed as solution a penetrative convection model with moderate overshooting parameter. Additional tests on Kepler's open clusters (NGC6791 and NGC6819) and secondary clump stars, allowed me to revised to my mixing model
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
Characterisation of FG-type stars with an improved transport of chemical elements
Context. The modelling of chemical transport mechanisms is crucial for
accurate stellar characterizations. Atomic diffusion is one of these processes
and it is commonly included in stellar models. However, it is usually neglected
for F-type or more massive stars because it produces surface abundance
variations that are unrealistic. Additional mechanisms to counteract atomic
diffusion must therefore be considered. It has been demonstrated that turbulent
mixing can prevent the surface abundance over-variations, and can also be
calibrated to mimic the effects of radiative accelerations on iron. Aims. We
aim to evaluate the effect of a calibrated turbulent mixing on the
characterisation of a sample of F-type stars, and how the estimates compare
with those obtained when the chemical transport mechanisms are neglected.
Methods. We selected stars from two samples - one from the Kepler LEGACY sample
and the other from a sample of Kepler planet-hosting stars. We inferred their
stellar properties using two grids. The first grid considers atomic diffusion
only in models that do not show chemical over-variations at the stellar
surface. The second grid includes atomic diffusion in all the stellar models
and the calibrated turbulent mixing to avoid unrealistic surface abundances.
Results. Comparing the derived results from the two grids, we found that the
results for the more massive stars in our sample will have higher dispersion in
the inferred values of mass, radius and age, due to the absence of atomic
diffusion in one of the grids. This can lead to relative uncertainties for
individual stars of up to 5% for masses, 2% for radii and 20% for ages.
Conclusions. This work shows that a proper modelling of the microscopic
transport processes is key for an accurate estimation of their fundamental
properties not only for G-type stars, but also for F-type stars.Comment: 19 pages, 13 figures, accepted for publication in A&
Atomic diffusion and turbulent mixing in solar-like stars: Impact on the fundamental properties of FG-type stars
Chemical composition is an important factor that affects stellar evolution.
The element abundance on the stellar surface evolves along the lifetime of the
star because of transport processes, including atomic diffusion. However,
models of stars with masses higher than about 1.2Msun predict unrealistic
variations at the stellar surface. This indicates the need for competing
transport processes that are mostly computationally expensive for large grids
of stellar models. The purpose of this study is to implement turbulent mixing
in stellar models and assess the possibility of reproducing the effect of
radiative accelerations with turbulent mixing for elements like iron in order
to make the computation of large grids possible. We computed stellar models
with MESA and assessed the effects of atomic diffusion (with radiative
acceleration) in the presence of turbulent mixing. We parametrised the effect
of radiative accelerations on iron with a turbulent diffusion coefficient.
Finally, we tested this parametrisation by modelling two F-type stars of the
Kepler Legacy sample. We found that, for iron, a parametrisation of turbulent
mixing that simulates the effect of radiative acceleration is possible. This
leads to an increase in the efficiency of the turbulent mixing to counteract
the effect of gravitational settling. This approximation does not affect
significantly the surface abundances of the other elements we studied, except
for oxygen and calcium. We demonstrate that this parametrisation has a
negligible impact on the accuracy of the seismic properties inferred with these
models. Moreover, turbulent mixing makes the computation of realistic F-type
star models including the effect atomic diffusion possible. This leads to
differences of about 10% in the inferred ages compared to results obtained with
models that neglect these processes.Comment: 16 pages, 12 figures, accepted for publication in A&
A synthetic sample of short-cadence solar-like oscillators for TESS
NASA's Transiting Exoplanet Survey Satellite (TESS) has begun a two-year
survey of most of the sky, which will include lightcurves for thousands of
solar-like oscillators sampled at a cadence of two minutes. To prepare for this
steady stream of data, we present a mock catalogue of lightcurves, designed to
realistically mimic the properties of the TESS sample. In the process, we also
present the first public release of the asteroFLAG Artificial Dataset
Generator, which simulates lightcurves of solar-like oscillators based on input
mode properties. The targets are drawn from a simulation of the Milky Way's
populations and are selected in the same way as TESS's true Asteroseismic
Target List. The lightcurves are produced by combining stellar models,
pulsation calculations and semi-empirical models of solar-like oscillators. We
describe the details of the catalogue and provide several examples. We provide
pristine lightcurves to which noise can be added easily. This mock catalogue
will be valuable in testing asteroseismology pipelines for TESS and our methods
can be applied in preparation and planning for other observatories and
observing campaigns.Comment: 14 pages, 6 figures, accepted for publication in ApJS. Archives
containing the mock catalogue are available at
https://doi.org/10.5281/zenodo.1470155 and the pipeline to produce it at
https://github.com/warrickball/s4tess . The first public release of the
asteroFLAG Artificial Dataset Generator v3 (AADG3) is described at
https://warrickball.github.io/AADG3
Determining stellar parameters of asteroseismic targets: Going beyond the use of scaling relations
Asteroseismic parameters allow us to measure the basic stellar properties of field giants observed far across the Galaxy. Most of such determinations are, up to now, based on simple scaling relations involving the large-frequency separation, \u394\u3bd, and the frequency of maximum power, \u3bdmax. In this work, we implement \u394\u3bd and the period spacing, \u394P, computed along detailed grids of stellar evolutionary tracks, into stellar isochrones and hence in a Bayesian method of parameter estimation. Tests with synthetic data reveal that masses and ages can be determined with typical precision of 5 and 19 per cent, respectively, provided precise seismic parameters are available. Adding independent on the stellar luminosity, these values can decrease down to 3 and 10 per cent, respectively. The application of these methods to NGC 6819 giants produces a mean age in agreement with those derived from isochrone fitting, and no evidence of systematic differences between RGB and RC stars. The age dispersion of NGC 6819 stars, however, is larger than expected, with at least part of the spread ascribable to stars that underwent mass-transfer events
TESS asteroseismology of the known red-giant host stars HD 212771 and HD 203949
International audienc
TESS Giants Transiting Giants V -- Two hot Jupiters orbiting red-giant hosts
In this work we present the discovery and confirmation of two hot Jupiters
orbiting red-giant stars, TOI-4377 b and TOI-4551 b, observed by TESS in the
southern ecliptic hemisphere and later followed-up with radial-velocity (RV)
observations. For TOI-4377 b we report a mass of $0.957^{+0.089}_{-0.087} \
M_\mathrm{J}1.348 \pm 0.081 \ R_\mathrm{J}1.36 \ \mathrm{M}_\odot3.52 \
\mathrm{R}_\odot4.3781.49 \pm 0.13 \ M_\mathrm{J}1.058^{+0.110}_{-0.062} \ R_\mathrm{J}1.31 \ \mathrm{M}_\odot3.55 \
\mathrm{R}_\odot9.9561.91 \pm 0.48\%2.19 \pm 0.45\%$ for
TOI-4377 b and TOI-4551 b respectively. These values are in line with the known
population of hot Jupiters, including hot Jupiters orbiting main sequence
hosts, which suggests that the radii of our planets have reinflated in step
with their parent star's brightening as they evolved into the
post-main-sequence. Finally, we evaluate the potential to observe orbital decay
in both systems.Comment: 14 pages with 8 figures and 6 tables. Accepted for publication in the
Monthly Notices of the Royal Astronomical Societ
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