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In this work, the emphasis will be placed on the understating of the theory needed to try
to describe correctly the interaction between convection and pulsation. We will shed light on
important terms appearing due to turbulent flows and see how we can describe them. Hence,
we will present more evolved models that should reproduce the reality with a better fidelity.
Finally, to illustrate and compare the different possible models introduced previously, we will
study a well-known type of variable stars: the ZZ Ceti white dwarfs. These dying stars are
known for their extremely thin but efficient convection envelope (as most white dwarfs) and
for their instability strip for which better predictions are still needed in order to match the
observations. Dans ce travail, l'emphase sera mise sur la compréhension de la théorie nécessaire pour décrire correctement l’interaction entre la convection et les pulsations. Nous mettrons en évidence l'apparition de termes importants supplémentaires provenant des écoulements turbulents et nous verrons comment ils peuvent être décrits. De plus, nous présenterons des modèles plus évolués censés reproduire la réalité avec une meilleure fidélité. Finalement, pour illustrer et comparer les différents modèles que nous auront préalablement introduits, nous étudierons un type d'étoiles variables bien connu : les naines blanches ZZ Ceti. Ces étoiles en fin de vie sont connues pour leur enveloppe convective fine mais très importante et surtout pour leur bande d'instabilité pour laquelle de meilleures prédictions sont toujours nécessaires afin de reproduire les observations.

Stars: oscillations --- Stars: convection --- White dwarfs --- Stars: interiors --- Etoiles: oscillations --- Etoiles: convection --- Naines blanches --- Etoiles: intérieurs --- Physique, chimie, mathématiques & sciences de la terre > Aérospatiale, astronomie & astrophysique

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The red giant branch bump is a phenomenon that consists of a decrease in luminosity for a given time during the red giant branch phase. Discrepancies between models and observations show that it is not yet fully understood. It is therefore interesting to try to understand the influence of various parameters that are part of the structure of the star on the bump. We have used the stellar evolution code called Clés to study the influence of the total mass of the star, its overshooting and undershooting parameters, its metallicity, and the microscopic and turbulent diffusion. To study the effects of changing these parameters, the attention was given to the track of the star in the Hertzsprung-Russell diagram during its evolution and on the hydrogen abundance profile of the star, the discontinuity in the latter being the key element for the appearance of the bump. Effects can be seen on the bump for each of the parameters studied, which can be summarized in two categories: a change in the luminosity at which the bump occurs and a change in the size of the bump, thus the importance of the observed decrease in luminosity.

Red Giant Branch Bump --- Stellar evololution --- Stellar structure --- Stellar physics --- red giant branch --- Physique, chimie, mathématiques & sciences de la terre > Aérospatiale, astronomie & astrophysique

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In the past decade, space missions, such the CoRoT, Kepler and TESS, have provided the asteroseismic community with a vast amount of observations, of unprecedented quality. This gave the opportunity to stellar astrophysicists, to study the stellar interiors of a large variety of pulsating stars. Amongst them, are the solar-like stars, that are low-mass main-sequence stars, reaching up to 2.3 Solar Masses. 
During the main-sequence phase, the solar-like stars are stable, and they present frequencies that are very regular, which is most obvious in a power spectrum. Nevertheless, there are some departures from that equidistance, to which we refer as the smooth part, and one of the reasons for that, are the glitches. The glitches are due to sharp variations, on the internal structure of the stars, which introduce an oscillatory feature, as a function of the frequency, in the oscillation spectrum. Solar-like stars experience mostly two glitches, the helium glitch, for which our study is about, and the convection zone glitch. The former is located in the second ionization zone of helium, quite close to the stellar surface, and the latter, is located in the transition region between the convective envelope and the radiation zone. The study of those glitches is important, because they provide information about their respective regions, which further helps constrain the internal structure of a star. The amplitude of the helium glitch, is directly linked to the helium abundance, in that area and, since that area is very close to the surface, and convection dominates there, it provides information about the surface helium abundance. This is the only way to obtain that value, because the stellar surface of solar-like stars, does not have the required temperature for the excitation of helium, and thus, few or no emission lines can be obtained spectroscopically. 
There exists a number of methods that use the observed frequencies, and derive seismic constraints, which are used to derive information about the stellar structure, through stellar modeling. The one that we used for our analysis, is the Whole Spectrum and Glitches Adjustment (WhoSGlAd). It uses both the smooth part of the spectrum, along with the glitches part, to derive seismic indicators, as little correlated as possible. The accuracy of those indicators, allows them to be used as constraints by minimization techniques, in order to retrieve precise values about global quantities, as is the mass, the age and the chemical composition. 
In the case of WhoSGlAd, one of the constraints that is used is the helium glitch amplitude, but for its value we need to know the helium acoustic depth. When we work with observations this value is not available and so, we have to derive it by a model, to estimate the helium acoustic depth, which takes time and it is inconvenient, since it makes the results model dependent. In this study, we propose a method, that uses a linear relation, in order to derive this acoustic depth, which uses only observed quantities and so, makes the procedure much faster. We prove the efficiency and the accuracy of that method, by using both models and observations. This means that it does not make any sacrifices in the precision of WhoSGlAd. Moreover, since this method is linear, and uses only observables, it can be implemented in already existing codes, and provide results much faster. The method, as a result of the assumed linear formulation, does not apply to cases that are not linear. This occurs in cases beyond the main sequence, or with convective cores. This can happen for example, for the case of stars ≥1.2 solar masses, for some chemical compositions.

Physique, chimie, mathématiques & sciences de la terre > Aérospatiale, astronomie & astrophysique