Doctoral thesis (Dissertations and theses)
Asteroseismic study of solar-like pulsators along their evolution
Farnir, Martin
2021
 

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Keywords :
Asterosismology; stars:oscillations; stars:solar-type; stars:low-mass; methods:numerical; stars:giants
Abstract :
[en] Following the advent of space-borne missions (e.g. CoRoT, Kepler), came a wealth of data of unprecedented quality. This enabled asteroseismology to thrive and to probe the stellar structure of a wide variety of pulsating stars. Amongst these pulsating stars is the notable category of low-mass stars. These exhibit masses below 2.3 solar-mass , encompassing the case of our Sun. Throughout their evolution, these stars exhibit a few interesting peculiarities. First, during the main-sequence phase, they display a very regular pressure-mode oscillation spectrum. However, small perturbations to that regularity may occur. Such perturbations are the result of sharp and localised variations in the stellar structure. These create an oscillating feature, as a function of the frequency, in the oscillation spectrum, the so-called glitches. These glitches are of particular interest as they allow us to probe very localised regions of the stellar interior and provide diagnoses about specific stellar features, inaccessible by other means. In main-sequence low-mass stars, we distinguish two main causes of glitches: the helium second-ionisation zone, providing information about the surface helium abundance, and the base of the envelope convection zone, constraining the mixing processes in that region. The first part of my thesis was dedicated to the development of a seismic technique, WhoSGlAd, that consistently analyses the complete oscillation spectra of main-sequence low-mass stars and robustly retrieves the glitches signatures present in these spectra. Special care was put in the definition of stringent seismic indicators as we decorrelated them as much as possible. This is done thanks to a Gram-Schmidt orthonormalisation process. The defined indicators were then used to constrain stellar models and provide a characterisation of both the 16 Cygni system and the Kepler Legacy Sample, representing the best solar-like seismic data currently available. After the main-sequence phase, low-mass stars evolve on the subgiant and red-giant phases. Their core then contracts while their envelope expands, developing a large core-envelope density contrast. This produces the appearance of mixed-modes, presenting a twofold nature: a gravity- dominated nature in the inner radiative regions, and a pressure-dominated nature in the outer convective regions. These modes have the great advantage to propagate throughout most of the stellar interior and, therefore, to probe almost the complete stellar structure. To exploit the information these modes carry, we developed the EGGMiMoSA method. It relies on the asymptotic expression and allows us to precisely measure seismic indicators on both subgiant and red-giant stars. The method was applied to a grid of models extending from the subgiant phase to the luminosity bump. The results are excellent in regard to the asymptotic values of the seismic indicators and also qualitatively agree with observed and theoretical studies. These indicators also allow us to efficiently infer the stellar age, mass, and radius of subgiant stars and of red-giant stars with masses & 1.8 M . Below this threshold, we noted that the central electron degeneracy impaired our diagnosis of the stellar age, mass, and radius in red-giants. The combination of both methods should provide means to constrain the stellar structure of low-mass stars from the early main-sequence phase to the late red-giant one. This is a unique opportunity to study their structure through most of their evolution and, for example, pinpoint missing physical processes in their modelling.
Research center :
STAR - Space sciences, Technologies and Astrophysics Research - ULiège
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Farnir, Martin ;  Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Astrophysique stellaire théorique et astérosismologie
Language :
English
Title :
Asteroseismic study of solar-like pulsators along their evolution
Defense date :
14 September 2021
Number of pages :
xii,182
Institution :
ULiège - Université de Liège
Degree :
Doctor of Philosophy in Science
Promotor :
Dupret, Marc-Antoine ;  Université de Liège - ULiège > Unités de recherche interfacultaires > Space sciences, Technologies and Astrophysics Research (STAR)
President :
Beckers, Jean-Marie  ;  Université de Liège - ULiège > Freshwater and OCeanic science Unit of reSearch (FOCUS)
Secretary :
Van Grootel, Valérie  ;  Université de Liège - ULiège > Unités de recherche interfacultaires > Space sciences, Technologies and Astrophysics Research (STAR)
Jury member :
Cunha, Margarida
Miglio, Andrea
Deheuvels, Sébastien
Goupil, Marie-Jo
Funders :
FRIA - Fonds pour la Formation à la Recherche dans l'Industrie et dans l'Agriculture [BE]
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since 01 September 2021

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