The space mission era turned asteroseismology into a tried and tested method on large scale. While Kepler has allowed us to climb the temperature scale from solar-like pulsators up to Slowly Pulsating B stars, TESS is now boosting us to even higher masses. In particular, the OB star regime (> 8M_sun), where asteroseismic calibration is direly needed. Stars in this high mass regime are born with convective cores which essentially drive their evolution, on and beyond the main sequence. Yet, the size of this convective core remains uncalibrated at different evolutionary stages due to uncertainties in the physical prescriptions of processes such as core overshooting, interior rotation and envelope mixing. In this seminar I present new results on the large OB asteroseismic sample observed with TESS. The sample is characterised by a diverse variability, including pulsations, binarity, and rotational modulation connected to spots induced by magnetic fields. In a first instance I present the diverse photometric and spectroscopic variability of these stars, and discuss their placement in theoretical instability strips computed using the stellar evolution code MESA and the stellar pulsation code GYRE. This exercise shows that many stars that are pulsating in a particular way are not predicted to do so, which points to missing physics. This includes rotation, which is important in the OB star regime, as many stars are rotating at a significant fraction of their critical rotation rate. To investigate this further, I discuss the analysis of high-mass pulsator HD192575. This early-type star (B0.5 V) shows rotation modulation and a number of frequency multiplets induced by rotation. I present a detailed spectroscopic and photometric analysis using HERMES spectroscopy and the 1-year TESS light curve. This analysis allows us to delimit the parameter space in the Hertzsprung-Russell diagram, in which we calculated a dense grid of pulsation models with varying degrees of mixing using the MESA and GYRE codes. We derived constraints on the age and mass of the star, as well as its interior rotation and mixing properties by using an estimator which includes theoretical uncertainties in the equilibrium models. These asteroseismic solutions further provide evidence of non-rigid rotation in HD192575. I conclude by discussing the further application of the method to other high-mass pulsators in our sample.
The success of the CoRoT and Kepler space-borne missions has opened a new era for stellar physics. Indeed, thanks to the four-year time series of Kepler, it is now possible to decipher in detail the oscillation spectrum of evolved giants. The information contained in these oscillation spectra, which still needs to be precisely studied for evolved red-giant branch (RGB) and asymptotic-giant branch (AGB) stars, is of major importance to probe the inner stellar structure, which is poorly probed with spectroscopy alone. In this context, we considered the acoustic modes that develop in stellar interiors and we performed a thorough analysis of the oscillation spectra of ~ 2.000 evolved giants, including RGB and AGB stars. My presentation will focus on the physical differences between RGB and AGB stars. I will highlight the significant differences in the typical signature of the second helium ionisation zone in mode frequencies, which makes a seismic classification between evolved RGB and AGB stars possible. The physical conditions inside those stars will be addressed with stellar evolution and oscillation codes.
Current asteroseismic catalogues are dominated by detections made in Kepler data. While the mission produced large numbers of evolved red giants and a substantial set of main sequence stars, observational constraints hampered detections in subgiants and low luminosity giants. To fill this gap we require larger surveys like TESS and the upcoming Plato mission. This introduces a new issue, how do we locate solar-type oscillators in enormous data sets. In an attempt to address these two issues, I will present a new automated detection pipeline designed for Plato and the catalogue of solar-like oscillators that fell out of our work testing it with TESS. These stars sit at the sharp cut on the RGB enforced by the Kepler long cadence nyquist frequency, and extend down the subgiant branch toward the main sequence.
The data from the Kepler nominal mission have enabled the detection of gravito-inertial modes in sequences of consecutive radial order in about 600 γ Doradus (γ Dor) pulsators. The fact that these modes are sensitive to the deep interior of stars, makes γ Dor pulsators excellent candidates for improving the current incomplete prescriptions of the transport of angular momentum and chemical elements, both important ingredients in stellar evolution theory. To that extent, forward modelling of gravito-inertial modes to derive stellar masses, ages, and mixing efficiencies throughout the radiative envelope, helps us to understand the possible origins of these transport mechanisms in intermediate-mass main-sequence stars. High-dimensional grid-based forward modelling quickly becomes computationally unfeasible for a large sample of stars due to the large number of required models. This can be circumvented by training neural networks on a set of stellar evolution and pulsation models to interpolate the observables in a high-dimensional parameter space. I will present a novel technique for modelling gravito-inertial modes in γ Dor pulsators, using deep learning, which relies on the individual observed mode periods, combined with spectroscopy, and Gaia luminosities as input. This method is combined with a framework which considers the correlated nature of the pulsations. The extracted (core) masses, ages, and core-overshooting parameters, combined with the rotation rates, allows for the investigation of any correlations present in the sample. Furthermore, I will discuss the effect of adding atomic diffusion (including radiative levitation) in the stellar models on the predicted pulsation frequencies, and compare the predicted surface abundances with observed ones.
We present an in-depth analysis of TESS observations of the pre-main sequence \delta Scuti binary RS Cha. RS Cha is an eclipsing binary system with well known stellar parameters that is synchronised and circularised. In a such a system, the equilibrium tide is expected to perturb the self-excited pulsation modes.
We used PHOEBE to remove the eclipsing binary signal from the TESS light curve to model the residuals with superposition of linear modes. Our investigation of the pulsations uncover mulitplets split by (twice) the orbital frequencies. We use the theory of tidally perturbed modes to identify the pulsation modes and verify previous spectroscopic mode identification for a prominent l=2 or l=3 mode.
Our results suggest that RS Cha is an ideal candidate to test the theory of tidally perturbed modes within the framework of asteroseismic modelling.
Furthermore, it is the first pre-main sequence object showing tidal effects on its pulsation spectrum. As such, RS Cha will be a keystone in the future of pre-main sequence asteroseismology.
Convective overshoot mixing has been shown to be an important ingredient of stellar structure models. It determines to a large degree how much fuel convective cores can consume, which strongly impacts on stellar luminosities and lifetimes. The convective flows encountered in stellar interiors are highly turbulent. This poses a number of numerical challenges for the modeling of convection in stars. In this talk I will discuss the theory and the results of an effective turbulence model which we implemented into a 1D stellar evolution code. The turbulent convection model in use relies on the solution of second order moment equations (e.g. for turbulent kinetic energy and turbulent convective flux). The dissipation due to buoyancy waves in the overshooting zone is taken into account in an approximate fashion. Applying this turbulent convection model we compute stellar evolution models of intermediate mass main-sequence stars between 2 and 8 solar masses. Phenomena like overshoot mixing and modified temperature gradients emerge naturally as a solution of the turbulent convection model equations. The overshooting extent determined from the turbulent convection model is comparable to other overshooting descriptions. The size of the mixed core decreases with decreasing stellar mass without external limitation. We find that the dissipation by buoyancy waves is a relevant ingredient in the turbulent convection model in use. High precision asteroseismic modeling will potentially allow testing some of the predictions made by this turbulent convection model.
Rotation plays an important role in the life of stars, and offers a potential diagnostic to infer their ages and that of their planets. This idea is known as gyrochronology, and while potentially fruitful over a wide range of ages and masses, recent results have raised concerns regarding its applicability. In the first part of this talk, I will discuss the impact that removing the non-member contamination, by using the precise Gaia astrometry, has on the rotational sequences of open clusters. The revised sequences demonstrate that ground-based periods can be as constraining as space-based periods, illustrate that stars in the 1.0-0.6 Msun range inhabit a global maximum in terms of rotation periods (with potential consequences for habitability), and reveal that in the saturated domain the rotational distributions broaden, in contradiction with predictions from popular models. In the second part of this talk, I will discuss the importance of subgiant stars as empirical tests for stellar rotation, focusing on a sample of TESS CVZs subgiants. Results from a detailed characterization study demonstrate that these stars are ideal targets for precise age and mass determinations based on HR diagram location alone. The complementary roles played by classical and asteroseismic analyses will place stringent constraints on angular momentum evolution theories.
Convective cores are the fuel reservoir of stars that are more massive than about 1.2 Msun. Their size therefore has a substantial influence on stellar evolution. However, several physical processes that remain poorly understood by theory can extend those convective cores. Observations are therefore required to help constrain them. In this talk, I will speak about how we can use subgiant asteroseismology to indirectly constrain the main-sequence convective core extension. Indeed, subgiant stars exhibit mixed modes, whose dual nature as pressure and gravity modes allows us to probe the very core of the star. We therefore used the full Kepler data set to thoroughly model KIC10273246, using a method that we specifically tailored for subgiants. We obtained models that show a good statistical agreement with the observations, and found that adding overshooting significantly improves the quality of the seismic fit. We also found that having access to several g-dominated mixed modes provides a stronger constraint on the structure of the star, especially the Brunt-Väisälä frequency and the central density. This study paves the way of a more general study, which will include subgiants observed with Kepler and TESS. Moreover, it led us to the study of the impact of other processes, such as convection and nuclear reactions, on the determination of convective cores size by the stellar evolution codes.
KIC 11145123 is a gamma Dor-delta Sct hybrid pulsator for which well-resolved frequency splittings for p, g, and mixed modes have been observed, leading to a number of detailed asteroseismic studies of the star such as stellar modeling and rotation inversion. All the previous models of the star have been constructed assuming single-star evolution. However, the star is spectroscopically considered a blue straggler, which is thought to have experienced some interactions with other stars. There is thus room for carrying out non-standard asteroseismic modeling of the star assuming non-single-star evolution. In this talk, we primarily discuss a non-standard model of the star. Comparison of the non-standard model with the previous models suggests that the star may well have experienced some modifications of chemical compositions in the envelope during the evolution, consistent with formation channels of blue straggler stars. We also would like to discuss the internal rotation profile of the star asteroseismically inferred based on the non-standard model. In particular, we focus on a detection of the fast-convective-core rotation in detail. A possible relation between the inferred fast-convective-core rotation and the structure in the deep radiative region of the star is investigated. Though we do not find a clear signature between the internal dynamics and structure of the star, this is the first study which tests the possibility that there is a relationship between the current rotational profile and the structure of stars other than the Sun, and to investigate such relationship should be one of the most highly prioritized subjects to future researches.
The Milky Way galaxy has been shaped by different processes throughout
its life, affecting its shining elements: the stars. Stars carry
signatures of the conditions in their birth environment and of their
subsequent history, meaning we can use the stars as
fossil records to reconstruct the evolution of the Milky Way. We can
also use the constraints on in-situ formation of external galaxies,
where individual stars cannot be resolved and this kind of study is
impossible.
One challenge is to achieve the necessary accuracy in stellar
parameters for a representative number of stars. Determining physical
properties of stars such as mass, radius, and especially age is
remarkably difficult using traditional techniques. In the
last few decades, the study of stellar pulsations or asteroseismology
has led to a dramatic development in the precise measure of stellar
parameters.
Stellar pulsations are directly related to the internal properties
of stars and thus to the nuclear processes in the stellar interior,
based on which relatively precise stellar ages can be determined.
I have compiled the largest ensemble of red giant stars to date with
measured elemental abundances, astrometric quantities, and precise ages
from asteroseismology. The aim of my project is to build a chronology of
the Milky Way galaxy and study the age aspect
of the chemical and kinematic features of the Galactic disk. Using a
data-driven clustering algorithm, I am looking at the similarities and
differences between stars born at different epochs, studying the
signatures of events in the Milky Way’s past.