Publications of Gaël Buldgen
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See detailAsteroseismic probing of low mass solar-like stars throughout their evolution with new techniques
Farnir, Martin ULiege; Dupret, Marc-Antoine ULiege; Buldgen, Gaël ULiege et al

Conference (2021, October 12)

In this oral contribution we present two new techniques that aim at precisely probing the stellar structure of low-mass solar-like stars. These two methods, that focus on different evolution stages (i.e ... [more ▼]

In this oral contribution we present two new techniques that aim at precisely probing the stellar structure of low-mass solar-like stars. These two methods, that focus on different evolution stages (i.e. the main-sequence stars, subgiants and red giants), provide reliable, accurate, fast and efficient means to tightly constrain the stellar structure through the definition of robust seismic indicators, which we proved to be excellent structural proxies. Indeed, they allow to precisely infer stellar masses, radii, ages and surface helium contents. This is particularly relevant to the field of exoplanetary science, as a precise determination of exoplanetary masses and radii relies on precise stellar properties. We will first present the potential of the WhoSGlAd method (Farnir et al. 2019) to accurately, and automatically, constrain the stellar structure of large samples of main-sequence stars, which is necessary in the context of the PLATO mission (Rauer et al. 2014). By building almost uncorrelated indicators defined to hold precise structural information, this method proposes a brand new approach to the adjustment of the oscillation spectra that these stars display. We will then present a new method to coherently account for the spectra of both sub-giant and red-giant stars, the EGGMiMoSA method (Farnir et al. 2021, submitted). Relying on the asymptotic description of mixed-modes (Shibahashi 1979, Mosser et al. 2012, Takata 2016), this is the first method that is able to follow the evolution of relevant seismic indicators during these phases, namely the period spacing, frequency separation, coupling factor and the pressure and gravity offsets, and therefore constrain the masses, radii and ages of these evolved stars. In addition, this method reliably provides measurements for these indicators in an automated fashion, which is a great opportunity for the broad characterisation of the large amount of data the PLATO mission is expected to generate. Finally, the combination of these two techniques, which are extremely fast, and their seismic indicators with large scales model search algorithms, such as AIMS (Rendle et al. 2019), could efficiently and robustly provide stellar masses, radii, ages and surface helium abundances for most of the stars observed by the PLATO spacecraft. [less ▲]

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See detailAsteroseismology of evolved stars to constrain the internal transport of angular momentum. IV. Internal rotation of Kepler 56 from an MCMC analysis of the rotational splittings
Fellay, L.; Buldgen, Gaël ULiege; Eggenberger, P. et al

in Astronomy and Astrophysics (2021)

The observations of global stellar oscillations of post main-sequence stars by space-based photometry missions allowed to directly determine their internal rotation. These constraints have pointed towards ... [more ▼]

The observations of global stellar oscillations of post main-sequence stars by space-based photometry missions allowed to directly determine their internal rotation. These constraints have pointed towards the existence of angular momentum transport processes unaccounted for in theoretical models. Constraining the properties of their internal rotation thus appears as the golden path to determine the physical nature of these missing dynamical processes. We wish to determine the robustness of a new approach to study the internal rotation of post main-sequence stars, using parametric rotation profiles coupled to a global optimization technique. We test our methodology on Kepler 56, a red giant observed by the Kepler mission. First, we carry out an extensive modelling of the star using global and local minimizations techniques, and seismic inversions. Then, using our best model, we study in details its internal rotation profile, we adopted a Bayesian approach to constrain stellar parametric predetermined rotation profiles using a Monte Carlo Markov Chain analysis of the rotational splittings of mixed modes. Our Monte Carlo Markov Chain analysis of the rotational splittings allows to determine the core and envelope rotation of Kepler 56 as well as give hints about the location of the transition between the slowly rotating envelope and the fast rotating core. We are able to discard a rigid rotation profile in the radiative regions followed by a power-law in the convective zone and show that the data favours a transition located in the radiative region, as predicted by processes originating from a turbulent nature. Our analysis of Kepler 56 indicates that turbulent processes whose transport efficiency is reduced by chemical gradients are favoured, while large scale fossil magnetic fields are disfavoured as a solution to the missing angular momentum transport. [less ▲]

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See detailGrids of stellar models with rotation. VI. Models from 0.8 to 120 M[SUB]⊙[/SUB] at a metallicity Z = 0.006
Eggenberger, Patrick; Ekström, Sylvia; Georgy, Cyril et al

in Astronomy and Astrophysics (2021), 652

Context. Grids of stellar models, computed with the same physical ingredients, allow one to study the impact of a given physics on a broad range of initial conditions and they are a key ingredient for ... [more ▼]

Context. Grids of stellar models, computed with the same physical ingredients, allow one to study the impact of a given physics on a broad range of initial conditions and they are a key ingredient for modeling the evolution of galaxies. <BR /> Aims: We present here a grid of single star models for masses between 0.8 and 120 M[SUB]⊙[/SUB], with and without rotation for a mass fraction of heavy element Z = 0.006, representative of the Large Magellanic Cloud (LMC). <BR /> Methods: We used the GENeva stellar Evolution Code. The evolution was computed until the end of the central carbon-burning phase, the early asymptotic giant branch phase, or the core helium-flash for massive, intermediate, and low mass stars, respectively. <BR /> Results: The outputs of the present stellar models are well framed by the outputs of the two grids obtained by our group for metallicities above and below the one considered here. The models of the present work provide a good fit to the nitrogen surface enrichments observed during the main sequence for stars in the LMC with initial masses around 15 M[SUB]⊙[/SUB]. They also reproduce the slope of the luminosity function of red supergiants of the LMC well, which is a feature that is sensitive to the time-averaged mass loss rate over the red supergiant phase. The most massive black hole that can be formed from the present models at Z = 0.006 is around 55 M[SUB]⊙[/SUB]. No model in the range of mass considered will enter into the pair-instability supernova regime, while the minimal mass to enter the region of pair pulsation instability is around 60 M[SUB]⊙[/SUB] for the rotating models and 85 M[SUB]⊙[/SUB] for the nonrotating ones. <BR /> Conclusions: The present models are of particular interest for comparisons with observations in the LMC and also in the outer regions of the Milky Way. We provide public access to numerical tables that can be used for computing interpolated tracks and for population synthesis studies. <P />The numerical data of the present grid are directly accessible on the web page at <A href="https://www.unige.ch/sciences/astro/evolution/fr/base-de-donnees/">https://www.unige.ch/sciences/astro/evolution/fr/base-de-donnees/</A>. [less ▲]

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See detailConvective core sizes in rotating massive stars: I. Constraints from solar metallicity OB field stars
Martinet, S.; Meynet, G.; Ekström, S. et al

in Astronomy and Astrophysics (2021), 648

Spectroscopic studies of Galactic O and B stars show that many stars with masses above 8 M$_{\odot}$ are observed in the HR diagram just beyond the Main-Sequence (MS) band predicted by stellar models ... [more ▼]

Spectroscopic studies of Galactic O and B stars show that many stars with masses above 8 M$_{\odot}$ are observed in the HR diagram just beyond the Main-Sequence (MS) band predicted by stellar models computed with a moderate overshooting. This may be an indication that the convective core sizes in stars in the upper part of the HR diagram are larger than predicted by these models. Combining stellar evolution models and spectroscopic parameters derived for a large sample of Galactic O and B stars, including brand new information about their projected rotational velocities, we reexamine the question of the convective core size in MS massive stars. We confirm that for stars more massive than about 8 M$_{\odot}$, the convective core size at the end of the MS phase increases more rapidly with the mass than in models computed with a constant step overshoot chosen to reproduce the main sequence width in the low mass range (around 2 M$_{\odot}$). This conclusion is valid for both the cases of non-rotating models and rotating models either with a moderate or a strong angular momentum transport. The increase of the convective core mass with the mass obtained from the TAMS position is, however, larger than the one deduced from the surface velocity drop for masses above about 15 M$_{\odot}$. Although observations available at the moment cannot decide what is the best choice between the core sizes given by the TAMS and the velocity drop, we discuss different methods to get out of this dilemma. At the moment, comparisons with eclipsing binaries seem to favor the solution given by the velocity drop. While we confirm the need for larger convective cores at higher masses, we find tensions in-between different methods for stars more massive than 15 M$_{\odot}$. The use of single-aged stellar populations (non-interacting binaries or stellar clusters) would be a great asset to resolve this tension. [less ▲]

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See detailGood vibrations: Low mass stars seismology with WhoSGlAd and EGGMiMoSA
Farnir, Martin ULiege; Dupret, Marc-Antoine ULiege; Pinçon, Charly ULiege et al

Conference (2021, March 10)

Since most of the information we receive from outer space is carried by stellar light, it comes as a necessity to properly characterise them. With the advent of space missions, it becomes possible to do ... [more ▼]

Since most of the information we receive from outer space is carried by stellar light, it comes as a necessity to properly characterise them. With the advent of space missions, it becomes possible to do so, thanks to the gathering of data of unprecedented quality. The past CoRoT and Kepler missions provided the stellar scientists with a wealth of data such that it allowed for the asteroseismology, the study of stellar pulsations and their link with the stellar structure, to thrive. This enabled to provide a detailed characterisation of distant solar-like stars and to pinpoint the shortcomings of current models. However, to provide precise inferences from observed oscillation spectra, it is necessary to have methods which are able to take the most advantage of the exquisite precision of the data. Both of the methods I develop are tailor-made for such needs. The first method, WhoSGlAd, accounts for the oscillations of main-sequence solar-like stars and the acoustic glitches they may exhibit. Acoustic glitches are an oscillating feature in the spectrum caused by a sharp feature in the stellar structure. The adjustment is done in such a way that the fitting parameters are completely independent and the computations are extremely fast. The parameters are then combined to build seismic indicators relevant of the stellar structure as little correlated as possible. Those are then used as constraints to stellar models. The second method, EGGMiMoSA, aims at providing a precise adjustment of the complex behaviour displayed by the mixed-modes oscillation spectra of sugbiant and red giant stars. Mixed-modes constitute a unique opportunity to probe the stellar interior from the surface to the core of the star. Again, the objective of the method is to define seismic indicators relevant of the stellar structure in order to constrain stellar models. During the present seminar, I will introduce both techniques and several results obtained via their use. [less ▲]

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See detailStandard solar models: Perspectives from updated solar neutrino fluxes and gravity-mode period spacing
Salmon, Sébastien ULiege; Buldgen, Gaël ULiege; Noels-Grötsch, Arlette ULiege et al

in Astronomy and Astrophysics (2021)

Context. Thanks to the vast and exquisite set of observations that have been made available for the Sun, our star is by far an ideal target for testing stellar models with a unique precision. A recent ... [more ▼]

Context. Thanks to the vast and exquisite set of observations that have been made available for the Sun, our star is by far an ideal target for testing stellar models with a unique precision. A recent issue under consideration in the field is related to the progress in the solar surface abundances derivation that has led to a decrease of the solar metallicity. While the former high-metallicity models were in fair agreement with other observational indicators from helioseismology and solar neutrino fluxes, it is no longer the case for low-metallicity models. This issue has become known as ’the solar problem’. Recent data are, however, promising to shed a new light on it. For instance, in 2020, the Borexino collaboration released the first-ever complete estimate of neutrinos emitted in the CNO cycle, which has reaffirmed the role of the neutrino constraints in the solar modelling process and their potential in exploring related issues. In parallel, a newly claimed detection of solar gravity modes of oscillation offers another opportunity for probing the stratification in the Sun’s central layers. Aims. We propose combining the diagnoses from neutrinos and helioseismology, both from pressure and gravity modes, in assessing the predictions of solar models. We compare in detail the different physical prescriptions currently at our disposal with regard to stellar model computations. Methods. We built a series of solar standard models based on a variation of the different physical ingredients directly affecting the core structure: opacity, chemical mixture, nuclear reactions rates. We compare the predictions of these models to their observational counterparts for the neutrinos fluxes, gravity-mode period spacing, and low-degree pressure mode frequency ratios. Results. The CNO neutrino flux confirms previous findings, exhibiting a preference for high-metallicity models. Nevertheless, we found that mild modification of the nuclear screening factors can re-match low-metallicity model predictions to observed fluxes, al- though it does not restore the agreement with the helioseismic frequency ratios. Neither the high-metallicity or low-metallicity models are able to reproduce the gravity-mode period spacing. The disagreement is huge, more than 100σ to the observed value. Reversely, the family of standard models narrows the expected range of the Sun’s period spacing: between ∼2150 to ∼2190 s. Moreover, we show this indicator can constrain the chemical mixture, opacity, and – to a lower extent – nuclear reactions in solar models. [less ▲]

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See detailReinvestigating α Cen AB in light of asteroseismic forward and inverse methods
Salmon, Sébastien ULiege; Van Grootel, Valérie ULiege; Buldgen, Gaël ULiege et al

in Astronomy and Astrophysics (2020)

The α Cen stellar system is the closest neighbour to our Sun. Its main component is a binary composed of two main-sequence stars, one more massive than the Sun and one less massive. The system's bright ... [more ▼]

The α Cen stellar system is the closest neighbour to our Sun. Its main component is a binary composed of two main-sequence stars, one more massive than the Sun and one less massive. The system's bright magnitude led to a wealth of astronomical observations over a long period, making it an appealing testbed for stellar physics. In particular, detection of stellar pulsations in both α Cen A and B has revealed the potential of asteroseismology for determining its fundamental stellar parameters. Asteroseismic studies have also focused on the presence of a convective core in the A component, but as yet without definitive confirmation. Progress in the determination of solar surface abundances and stellar opacities have yielded new input for stellar theoretical models. We investigate their impact on a reference system such as α Cen AB. We seek to confirm the presence of a convective core in α Cen A by analysing the role of different stellar physics and the potential of asteroseismic inverse methods. We present a new series of asteroseismic calibrations carried out using forward approach modelling and including updated chemical mixture and opacities in the models. We then complement our analysis with help of recent asteroseismic diagnostic tools based on inverse methods developed for solar-like stars. The inclusion of an updated chemical mixture -- that is less metal-rich -- appears to reduce the predicted asteroseismic masses of each component. Neither classical asteroseismic indicators such as frequency ratios, nor asteroseismic inversions favour the presence of a convective core in α Cen A. The quality of the observational seismic dataset is the main limiting factor to settle the issue. Implementing new observing strategies to improve the precision on the pulsation frequencies would certainly refine the outcome of asteroseismology for this binary system. [less ▲]

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See detailSeismic solar models from Ledoux discriminant inversions
Buldgen, Gaël ULiege; Eggenberger, P.; Baturin, V. A. et al

in Astronomy and Astrophysics (2020), 642

Context. The Sun constitutes an excellent laboratory of fundamental physics. With the advent of helioseismology, we were able to probe its internal layers with unprecendented precision and thoroughness ... [more ▼]

Context. The Sun constitutes an excellent laboratory of fundamental physics. With the advent of helioseismology, we were able to probe its internal layers with unprecendented precision and thoroughness. However, the current state of solar modelling is still stained by tedious issues. One of these central problems is related to the disagreement between models computed with recent photospheric abundances and helioseismic constraints. The observed discrepancies raise questions on some fundamental ingredients entering the computation of solar and stellar evolution models. <BR /> Aims: We used solar evolutionary models as initial conditions for reintegrating their structure using Ledoux discriminant inversions. The resulting models are defined as seismic solar models, satisfying the equations of hydrostatic equilibrium. These seismic models will allow us to better constrain the internal structure of the Sun and provide complementary information to that of calibrated standard and non-standard models. <BR /> Methods: We used inversions of the Ledoux discriminant to reintegrate seismic solar models satisfying the equations of hydrostatic equilibrium. These seismic models were computed using various reference models with different equations of state, abundances, and opacity tables. We checked the robustness of our approach by confirming the good agreement of our seismic models in terms of sound speed, density, and entropy proxy inversions, as well as frequency-separation ratios of low-degree pressure modes. <BR /> Results: Our method allows us to determine the Ledoux discriminant profile of the Sun with an excellent accuracy and compute full profiles of this quantity. Our seismic models show an agreement with seismic data of ≈0.1% in sound speed, density, and entropy proxy after seven iterations in addition to an excellent agreement with the observed frequency-separation ratios. They surpass all standard and non-standard evolutionary models including ad hoc modifications of their physical ingredients that aim to reproduce helioseismic constraints. <BR /> Conclusions: The obtained seismic Ledoux discriminant profile, as well as the full consistent structure obtained from our reconstruction procedure paves the way for renewed attempts at constraining the solar modelling problem and the missing physical processes acting in the solar interior by breaking free from the hypotheses of evolutionary models. [less ▲]

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See detailInvestigating surface correction relations for RGB stars
Jorgensen, Andreas Christ Solvsten; Montalban, Josefina; Miglio, Andrea et al

in Monthly Notices of the Royal Astronomical Society (2020), 495(4), 4965-4980

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See detailFrom the Sun to solar-like stars: how does the solar modelling problem affect our studies of solar-like oscillators?
Buldgen, Gaël ULiege

in Lovekin, C.C.; Moffat, A.F.J.; Neiner, C. (Eds.) et al Stars and their Variability Observed from Space (2020)

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See detailDetermination of precise stellar parameters of Kepler LEGACY targets using the WhoSGlAd method
Farnir, Martin ULiege; Dupret, Marc-Antoine ULiege; Salmon, Sébastien ULiege et al

in Neiner, Coralie; Weiss, Werner; Baade, D. (Eds.) et al Proceedings of the conference Stars and their Variability Observed from Space (2020)

We developed a method, WhoSGlAd, that provides a comprehensive adjustment of solar-like oscillationspectra. The method allows for tighter constraints (up to four times smaller standard deviations than ... [more ▼]

We developed a method, WhoSGlAd, that provides a comprehensive adjustment of solar-like oscillationspectra. The method allows for tighter constraints (up to four times smaller standard deviations than thoseof analogous ones). We take advantage of this new method and of the quality of theKeplerLEGACY datato highlight trends in the stellar parameters and the limitations of the current generation of stellar models. [less ▲]

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See detailThorough characterisation of the 16 Cygni system. I. Forward seismic modelling with WhoSGlAd
Farnir, Martin ULiege; Dupret, Marc-Antoine ULiege; Buldgen, Gaël ULiege et al

in Astronomy and Astrophysics (2020)

Context: Being part of the brightest solar-like stars, and close solar analogues, the 16 Cygni system is of great interest to the scientific community and may provide insight into the past and future ... [more ▼]

Context: Being part of the brightest solar-like stars, and close solar analogues, the 16 Cygni system is of great interest to the scientific community and may provide insight into the past and future evolution of our Sun. It has been observed thoroughly by the Kepler satellite, which provided us with data of an unprecedented quality. Aims: This paper is the first of a series aiming to extensively characterise the system. We test several choices of micro- and macro-physics to highlight their effects on optimal stellar parameters and provide realistic stellar parameter ranges. Methods: We used a recently developed method, WhoSGlAd, that takes the utmost advantage of the whole oscillation spectrum of solar-like stars by simultaneously adjusting the acoustic glitches and the smoothly varying trend. For each choice of input physics, we computed models which account, at best, for a set of seismic indicators that are representative of the stellar structure and are as uncorrelated as possible. The search for optimal models was carried out through a Levenberg-Marquardt minimisation. First, we found individual optimal models for both stars. We then selected the best candidates to fit both stars while imposing a common age and composition. Results: We computed realistic ranges of stellar parameters for individual stars. We also provide two models of the system regarded as a whole. We were not able to build binary models with the whole set of choices of input physics considered for individual stars as our constraints seem too stringent. We may need to include additional parameters to the optimal model search or invoke non-standard physical processes. [less ▲]

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See detailOpen problems in high-mass stellar evolution
Ekström, Sylvia; Meynet, Georges; Georgy, Cyril et al

in Neiner, C.; Weiss, W.W.; Baade, D. (Eds.) et al Stars and their Variability Observed from Space (2020)

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See detailGlobal Helioseismology
Buldgen, Gaël ULiege

in Bulletin de la Société Royale des Sciences de Liège (2019, December), 88

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See detailHow Much do we Trust Stellar Models? Foreword
Baglin, Annie; Buldgen, Gaël ULiege; Dupret, Marc-Antoine ULiege et al

in Bulletin de la Société Royale des Sciences de Liège (2019, December), 88(1-4),

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See detailAsteroseismology of evolved stars to constrain the internal transport of angular momentum. II. Test of a revised prescription for transport by the Tayler instability
Eggenberger, P.; den Hartogh, J. W.; Buldgen, Gaël ULiege et al

in Astronomy and Astrophysics (2019), 631

Context. Asteroseismic observations enable the characterisation of the internal rotation of evolved stars. These measurements reveal that an unknown efficient angular momentum (AM) transport mechanism is ... [more ▼]

Context. Asteroseismic observations enable the characterisation of the internal rotation of evolved stars. These measurements reveal that an unknown efficient angular momentum (AM) transport mechanism is needed for subgiant and red giant stars in addition to hydrodynamic transport processes. A revised prescription for AM transport by the magnetic Tayler instability has been recently proposed as a possible candidate for such a missing mechanism. <BR /> Aims: We compare the rotational properties predicted by this magnetic AM transport to asteroseismic constraints obtained for evolved stars with a particular focus on the subgiant phase. <BR /> Methods: We computed models accounting for the recent prescription for AM transport by the Tayler instability with the Geneva stellar evolution code for subgiant and red giant stars, for which an asteroseismic determination of both core and surface rotation rates is available. <BR /> Results: The revised prescription for the transport by the Tayler instability leads to low core rotation rates after the main sequence that are in better global agreement with asteroseismic measurements than those predicted by models with purely hydrodynamic processes or with the original Tayler-Spruit dynamo. A detailed comparison with asteroseismic data shows that the rotational properties of at most two of the six subgiants can be correctly reproduced by models accounting for this revised magnetic transport process. This result is obtained independently of the value adopted for the calibration parameter in this prescription. We also find that this transport by the Tayler instability faces difficulties in simultaneously reproducing asteroseismic measurements available for subgiant and red giant stars. The low values of the calibration parameter needed to correctly reproduce the rotational properties of two of the six subgiants lead to core rotation rates during the red giant phase that are too high. Inversely, the higher values of this parameter needed to reproduce the core rotation rates of red giants lead to a very low degree of radial differential rotation before the red giant phase, which is in contradiction with the internal rotation of subgiant stars. <BR /> Conclusions: In its present form, the revised prescription for the transport by the Tayler instability does not provide a complete solution to the missing AM transport revealed by asteroseismology of evolved stars. <P /> [less ▲]

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See detailRevisiting Kepler-444. I. Seismic modeling and inversions of stellar structure
Buldgen, Gaël ULiege; Farnir, Martin ULiege; Pezzotti, C. et al

in Astronomy and Astrophysics (2019), 630

Context. The CoRoT and Kepler missions have paved the way for synergies between exoplanetology and asteroseismology. The use of seismic data helps providing stringent constraints on the stellar properties ... [more ▼]

Context. The CoRoT and Kepler missions have paved the way for synergies between exoplanetology and asteroseismology. The use of seismic data helps providing stringent constraints on the stellar properties which directly impact the results of planetary studies. Amongst the most interesting planetary systems discovered by Kepler, Kepler-444 is unique by the quality of its seismic and classical stellar constraints. Its magnitude, age and the presence of 5 small-sized planets orbiting this target makes it an exceptional testbed for exoplanetology. <BR /> Aims: We aim at providing a detailed characterization of Kepler-444, focusing on the dependency of the results on variations of key ingredients of the theoretical stellar models. This thorough study will serve as a basis for future investigations of the planetary evolution of the system orbiting Kepler-444. <BR /> Methods: We use local and global minimization techniques to study the internal structure of the exoplanet-host star Kepler-444. We combine seismic observations from the Kepler mission, Gaia DR2 data, and revised spectroscopic parameters to precisely constrain its internal structure and evolution. <BR /> Results: We provide updated robust and precise determinations of the fundamental parameters of Kepler-444 and demonstrate that this low-mass star bore a convective core during a significant portion of its life on the main sequence. Using seismic data, we are able to estimate the lifetime of the convective core to approximately 8 Gyr out of the 11 Gyr of the evolution of Kepler-444. The revised stellar parameters found by our thorough study are M = 0.754 ± 0.03 M[SUB]☉[/SUB], R = 0.753 ± 0.01 R[SUB]☉[/SUB], and Age = 11 ± 1 Gyr. [less ▲]

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See detailWhoSGlAd : A precise characterisation of the Kepler Legacy sample
Farnir, Martin ULiege; Dupret, Marc-Antoine ULiege; Salmon, Sébastien ULiege et al

Conference (2019, August 20)

The increasing number of spaceborne missions provides us with a wealth of exquisite data. Such data combined we the new techniques that are developed then provide us with a deeper understanding of stellar ... [more ▼]

The increasing number of spaceborne missions provides us with a wealth of exquisite data. Such data combined we the new techniques that are developed then provide us with a deeper understanding of stellar structure and evolution but also shed light on the uncertainties of the current stellar models (e.g. angular momentum transport mechanisms, extent of mixed regions, chemical composition,...). Asteroseimology is one of those techniques. It links stellar pulsation frequencies to the stellar structure and allows the probing of stellar interiors. In particular, the observation of acoustic glitches, which consist in oscillating features in spectra and are caused by sharp variations of the stellar structure, provides very stringent information. Namely, they inform us on the localisation of the transition between a radiative and a convective region or the envelope helium abundance. Thus, the analysis of the glitches signal allows us to provide constraints on several model uncertainties. 25We present here a new method to analyse the oscillation spectra of solar-like pulsators as a whole, i.e. both the acoustic glitches and the slowly varying trend (smooth component) are treated simultaneously. The method, WhoSGlAd (Farnir et al. 2019), takes advantage of linear algebra to compute seismic indicators that are as uncorrelated as possible. Namely, the glitches indicators are completely independent of those of the smoothcomponent. Moreover, the indicators we build are more precise than the ‘usual’ ones which are defined as a function of the radial order. Therefore, it allows to put more stringent constraints on the stellar structure. As an example, we may retrieve an estimation of the surface helium abundance (inaccessible by other means) or obtain constrains on the necessary amount of core overshooting to reproduce observations. Those are more accurate than what we may derive from ‘classical’ means. Such precise measurements are necessary to lift model uncertainties (such as the mass –helium degeneracy observed by Lebreton & Goupil 2014). After showing the advantages and necessity of the method, we will present the application of the method to most stars of the Kepler LEGACY sample (Lund et al. 2017) via the use of the AIMS algorithm. This will enable us to draw trends in the surface helium abundance as well as necessary amount of core overshooting for solar-like pulsators. [less ▲]

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See detailProgress in Global Helioseismology: A New Light on the Solar Modeling Problem and Its Implications for Solar-Like Stars
Buldgen, Gaël ULiege; Salmon, Sébastien ULiege; Noels-Grötsch, Arlette ULiege

in Frontiers in Astronomy and Space Sciences (2019), 6

Since the first observations of solar oscillations in 1962, helioseismology has probably been one of the most successful fields of astrophysics. Data of unprecedented quality were obtained through the ... [more ▼]

Since the first observations of solar oscillations in 1962, helioseismology has probably been one of the most successful fields of astrophysics. Data of unprecedented quality were obtained through the implementation of networks of ground-based observatories such as the GONG project or the BiSON network, coupled with space-based telescopes such as SOHO, Solar Orbiter and SDO missions. Besides the improvement of observational data, solar seismologists developed sophisticated techniques to infer the internal structure of the Sun from its eigenfrequencies. These methods, then already extensively used in the field of Geophysics, are called inversion techniques. They allowed to precisely determine the position of the solar convective envelope, the helium abundance in this region and the internal radial profiles of given thermodynamic quantities. Back in 1990s these comparisons showed a very high agreement between solar models and the Sun. However, the downward revision of the CNO surface abundances in the Sun in 2005, confirmed in 2009, induced a drastic reduction of this agreement leading to the so-called solar modelling problem. More than ten years later, in the era of the space-based photometry missions which have established asteroseismology of solar-like stars as a standard approach to obtain their masses, radii and ages, the solar modelling problem still awaits a solution. In this paper, we will present the results of new helioseismic inversions, discuss the current uncertainties of solar models as well as some possible solutions to the solar modelling problem. We will show how helioseismology can help us grasp what is amiss in our solar models. We will also show that, far from being an argument about details of solar models, the solar problem has significant implications for seismology of solar-like stars, on the main sequence and beyond, impacting asteroseismology as a whole as well as the fields requiring precise and accurate knowledge of stellar masses, radii and ages, such as Galactic archaeology and exoplanetology. [less ▲]

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See detailRotation rate of the solar core as a key constraint to magnetic angular momentum transport in stellar interiors
Eggenberger, P.; Buldgen, Gaël ULiege; Salmon, Sébastien ULiege

in Astronomy and Astrophysics (2019), 626

Context. The internal rotation of the Sun constitutes a fundamental constraint when modelling angular momentum transport in stellar interiors. In addition to the more external regions of the solar ... [more ▼]

Context. The internal rotation of the Sun constitutes a fundamental constraint when modelling angular momentum transport in stellar interiors. In addition to the more external regions of the solar radiative zone probed by pressure modes, measurements of rotational splittings of gravity modes would offer an invaluable constraint on the rotation of the solar core. Aims. We study the constraints that a measurement of the core rotation rate of the Sun could bring on magnetic angular momentum transport in stellar radiative zones. Methods. Solar models accounting for angular momentum transport by hydrodynamic and magnetic instabilities were computed for different initial velocities and disc lifetimes on the pre-main sequence to reproduce the surface rotation velocities observed for solar-type stars in open clusters. The internal rotation of these solar models was then compared to helioseismic measurements. Results. We first show that models computed with angular momentum transport by magnetic instabilities and a recent prescription for the braking of the stellar surface by magnetized winds can reproduce the observations of surface velocities of stars in open clusters. These solar models predict both a flat rotation profile in the external part of the solar radiative zone probed by pressure modes and an increase in the rotation rate in the solar core, where the stabilizing effect of chemical gradients plays a key role. A rapid rotation of the core of the Sun, as suggested by reported detections of gravity modes, is thus found to be compatible with angular momentum transport by magnetic instabilities. Moreover, we show that the efficiency of magnetic angular momentum transport in regions of strong chemical gradients can be calibrated by the solar core rotation rate independently from the unknown rotational history of the Sun. In particular, we find that a recent revised prescription for the transport of angular momentum by the Tayler instability can be easily distinguished from the original Tayler-Spruit dynamo, with a faster rotating solar core supporting the original prescription. Conclusions. By calibrating the efficiency of magnetic angular momentum transport in regions of strong chemical gradients, a determination of the solar core rotation rate through gravity modes is of prime relevance not only for the Sun, but for stars in general, since radial differential rotation precisely develops in these regions during the more advanced stages of evolution. [less ▲]

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