22 research outputs found
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
Asteroseismic modelling strategies in the PLATO era I. Mean density inversions and direct treatment of the seismic information
Asteroseismic modelling will be part of the pipeline of the PLATO mission and
will play a key role in the mission precision requirements on stellar mass,
radius and age. It is therefore crucial to compare how current modelling
strategies perform, and discuss the limitations and remaining challenges for
PLATO, such as the so-called surface effects, the choice of physical
ingredients, and stellar activity. In this context, we carried out a systematic
study of the impact of surface effects on the estimation of stellar parameters.
In this work, we demonstrated how combining a mean density inversion with a fit
of frequencies separation ratios can efficiently damp the surface effects and
achieve precise and accurate stellar parameters for ten Kepler LEGACY targets,
well within the PLATO mission requirements.
We applied and compared two modelling approaches, directly fitting the
individual frequencies, or coupling a mean density inversion with a fit of the
ratios, to six synthetic targets with a patched 3D atmosphere from Sonoi et al.
(2015) and ten actual targets from the LEGACY sample. The fit of the individual
frequencies is unsurprisingly very sensitive to surface effects and the stellar
parameters tend to be biased, which constitutes a fundamental limit to both
accuracy and precision. In contrast, coupling a mean density inversion and a
fit of the ratios efficiently damps the surface effects, and allows us to get
both precise and accurate stellar parameters. The average statistical precision
of our selection of LEGACY targets with this second strategy is 1.9% for the
mass, 0.7% for the radius, and 4.1% for the age, well within the PLATO
requirements. Using the mean density in the constraints significantly improves
the precision of the mass, radius and age determinations, on average by 20%,
33%, and 16%, respectively.Comment: Accepted for publication in Astronomy & Astrophysic
Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars
Red giants are evolved stars that have exhausted the supply of hydrogen in
their cores and instead burn hydrogen in a surrounding shell. Once a red giant
is sufficiently evolved, the helium in the core also undergoes fusion.
Outstanding issues in our understanding of red giants include uncertainties in
the amount of mass lost at the surface before helium ignition and the amount of
internal mixing from rotation and other processes. Progress is hampered by our
inability to distinguish between red giants burning helium in the core and
those still only burning hydrogen in a shell. Asteroseismology offers a way
forward, being a powerful tool for probing the internal structures of stars
using their natural oscillation frequencies. Here we report observations of
gravity-mode period spacings in red giants that permit a distinction between
evolutionary stages to be made. We use high-precision photometry obtained with
the Kepler spacecraft over more than a year to measure oscillations in several
hundred red giants. We find many stars whose dipole modes show sequences with
approximately regular period spacings. These stars fall into two clear groups,
allowing us to distinguish unambiguously between hydrogen-shell-burning stars
(period spacing mostly about 50 seconds) and those that are also burning helium
(period spacing about 100 to 300 seconds).Comment: to appear as a Letter to Natur
Detection of solar-like oscillations from Kepler photometry of the open cluster NGC 6819
Asteroseismology of stars in clusters has been a long-sought goal because the
assumption of a common age, distance and initial chemical composition allows
strong tests of the theory of stellar evolution. We report results from the
first 34 days of science data from the Kepler Mission for the open cluster NGC
6819 -- one of four clusters in the field of view. We obtain the first clear
detections of solar-like oscillations in the cluster red giants and are able to
measure the large frequency separation and the frequency of maximum oscillation
power. We find that the asteroseismic parameters allow us to test
cluster-membership of the stars, and even with the limited seismic data in
hand, we can already identify four possible non-members despite their having a
better than 80% membership probability from radial velocity measurements. We
are also able to determine the oscillation amplitudes for stars that span about
two orders of magnitude in luminosity and find good agreement with the
prediction that oscillation amplitudes scale as the luminosity to the power of
0.7. These early results demonstrate the unique potential of asteroseismology
of the stellar clusters observed by Kepler.Comment: 5 pages, 4 figures, accepted by ApJ (Lett.
The SAPP pipeline for the determination of stellar abundances and atmospheric parameters of stars in the core program of the PLATO mission
We introduce the SAPP (Stellar Abundances and atmospheric Parameters Pipeline), the prototype of the code that will be used to determine parameters of stars observed within the core program of the PLATO space mission. The pipeline is based on the Bayesian inference and provides effective temperature, surface gravity, metallicity, chemical abundances, and luminosity. The code in its more general version has a much wider range of potential applications. It can also provide masses, ages, and radii of stars and can be used with stellar types not targeted by the PLATO core program, such as red giants. We validate the code on a set of 27 benchmark stars that includes 19 FGK-type dwarfs, 6 GK-type subgiants, and 2 red giants. Our results suggest that combining various observables is the optimal approach, as this allows the degeneracies between different parameters to be broken and yields more accurate values of stellar parameters and more realistic uncertainties. For the PLATO core sample, we obtain a typical uncertainty of 27 (syst.) ± 37 (stat.) K for Teff, 0.00 ± 0.01 dex for log g, 0.02 ± 0.02 dex for metallicity [Fe/H], −0.01 ± 0.03 R⊙ for radii, −0.01 ± 0.05 M⊙ for stellar masses, and −0.14 ± 0.63 Gyr for ages. We also show that the best results are obtained by combining the νmax scaling relation with stellar spectra. This resolves the notorious problem of degeneracies, which is particularly important for F-type stars