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 ($\Delta \Pi_1$) of red-giant stars that ignite He burning in degenerate conditions (M$\lesssim$2.0 M$_{\odot}$). We also compare the predictions of these models with Kepler observations. For a given $\Delta\nu$, $\Delta \Pi_1$ 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 $\Delta \Pi_1$ can provide information as to their stellar luminosity, within ~10-20%. In general, the trends of $\Delta \Pi_1$ 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

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 $0.957^{+0.089}_{-0.087} \ M_\mathrm{J}$and a inflated radius of$1.348 \pm 0.081 \ R_\mathrm{J}$orbiting an evolved intermediate-mass star ($1.36 \ \mathrm{M}_\odot$,$3.52 \ \mathrm{R}_\odot$; TIC 394918211) on a period of of$4.378$days. For TOI-4551 b we report a mass of$1.49 \pm 0.13 \ M_\mathrm{J}$and a radius that is not obviously inflated of$1.058^{+0.110}_{-0.062} \ R_\mathrm{J}$, also orbiting an evolved intermediate-mass star ($1.31 \ \mathrm{M}_\odot$,$3.55 \ \mathrm{R}_\odot$; TIC 204650483) on a period of$9.956$days. We place both planets in context of known systems with hot Jupiters orbiting evolved hosts, and note that both planets follow the observed trend of the known stellar incident flux-planetary radius relation observed for these short-period giants. Additionally, we produce planetary interior models to estimate the heating efficiency with which stellar incident flux is deposited in the planet's interior, estimating values of$1.91 \pm 0.48\%$and$2.19 \pm 0.45\%$ 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

Revisiting the Red Giant Branch Hosts KOI-3886 and ι Draconis.:Detailed Asteroseismic Modeling and Consolidated Stellar Parameters

Asteroseismology is playing an increasingly important role in the characterization of red giant host stars and their planetary systems. Here, we conduct detailed asteroseismic modeling of the evolved red giant branch (RGB) hosts KOI-3886 and ι Draconis, making use of end-of-mission Kepler (KOI-3886) and multisector TESS (ι Draconis) time-series photometry. We also model the benchmark star KIC 8410637, a member of an eclipsing binary, thus providing a direct test to the seismic determination. We test the impact of adopting different sets of observed modes as seismic constraints. Inclusion of .,"= 1 and 2 modes improves the precision of the stellar parameters, albeit marginally, compared to adopting radial modes alone, with 1.9%-3.0% (radius), 5%-9% (mass), and 19%-25% (age) reached when using all p-dominated modes as constraints. Given the very small spacing of adjacent dipole mixed modes in evolved RGB stars, the sparse set of observed g-dominated modes is not able to provide extra constraints, further leading to highly multimodal posteriors. Access to multiyear time-series photometry does not improve matters, with detailed modeling of evolved RGB stars based on (lower-resolution) TESS data sets attaining a precision commensurate with that based on end-of-mission Kepler data. Furthermore, we test the impact of varying the atmospheric boundary condition in our stellar models. We find the mass and radius estimates to be insensitive to the description of the near-surface layers, at the expense of substantially changing both the near-surface structure of the best-fitting models and the values of associated parameters like the initial helium abundance, Y i . Attempts to measure Y i from seismic modeling of red giants may thus be systematically dependent on the choice of atmospheric physics