Lisa Bugnet

Asteroseismic estimates of the magnetic field strength in the radiative interior of red giant stars depend strongly on the internal stellar structure derived from models. Since red-giant-branch merger products have been shown to be able to possess a different core structure than single stars of the same mass, we investigated how a mass-gain history influences our estimates of an internal magnetic field strength. We constructed stellar models with and without a mass-gain event after the onset of the red giant branch evolutionary phase with masses of 1.1 M _⊙ ≤ M ≤ 2 M _⊙ . First, by assuming a weak magnetic field, we investigated the influence of a mass-gain event on the global sensitivity of the oscillation frequencies to the magnetic field. We find that mass-gain models can be several times more sensitive to the field than single-star models of identical total mass at masses above 1.6 M _⊙ . Therefore, considering a mass-gain evolutionary history for merger candidates allows a significant correction to the magnetic field strength. In the presence of strong magnetic fields, we also show that the critical field strength needed to suppress mixed dipole modes is significantly lower if a mass-gain event has occurred (for stars with masses of M ≳ 1.6 M _⊙ ). The massive end of the suppressed stars’ distribution is therefore strongly favored by a merger origin. We conclude that properly constraining the stellar evolutionary history is essential when aiming to constrain internal field strength estimates from asteroseismic observations.

Context. Magnetic fields in the radiative interiors of red giants are measured using shifts in the stellar oscillation frequencies. In the asymptotic framework, however, converting an observed frequency shift into a radial magnetic field strength requires knowing the global magnetic sensitivity. The parameter that describes this sensitivity (also called the core structure parameter) must be inferred from stellar models, which introduces a potential source of uncertainty. Aims. We seek to understand how the global magnetic sensitivity depends on stellar properties such as mass and metallicity, and to quantify the model-based uncertainty on magnetic field measurements. We also explore the stellar properties that are key for a precise and accurate estimate of the global magnetic sensitivity parameter. Methods. Using MESA models, we examined how the global magnetic sensitivity changes with mass, metallicity, and age. We then created a sample of synthetic stars and tested how well our grid-based fitting method recovered the sensitivity parameter. We considered different grid construction approaches and the choice of which observational properties are used in the fitting process. Results. The global magnetic sensitivity depends more strongly on mass for higher-mass models and the metallicity dependence is stronger for lower-metallicity models. Our fitting methods recover the underlying sensitivity parameter well, with a model-based uncertainty of 10% when precise metallicity measurements were used. We applied our procedure to stars with existing magnetic field measurements. In most cases, the dominant source of uncertainty remains observational, although such precise modeling can significantly reduce the magnetic field uncertainty for stars with exceptional data. Conclusions. With careful fitting, stellar models yield accurate values for the global magnetic sensitivity. We recommend that future work obtain the global magnetic sensitivity using asteroseismic and high-quality spectroscopic constraints. Under these conditions, we recommend adopting a model-based uncertainty of 10% on the sensitivity parameter.

The detection of strong, large-scale magnetic fields at the surfaces of the oldest white dwarfs might point toward a hidden internal magnetic field slowly rising to the surface. In addition, strong magnetic fields have recently been measured through asteroseismology in the radiative interiors of red giant stars, the progenitors of white dwarfs. To investigate the potential connection between these observations, we revisited the fossil field framework using asteroseismic detections to constrain the strength of such magnetic fields as red giants evolve into the white dwarf stage. We assumed that the magnetic field was either created during the core convection on the main sequence or that it fills the radiative interior as the star evolves on the red giant branch. From these initial conditions, we evolved the magnetic flux, allowing for magnetic diffusion along the evolution of a modeled 1.5 M _⊙ star. We find that measured field strengths in red giants attributed to the hydrogen-burning shell are compatible with the field amplitudes and emergence timescales of magnetized white dwarfs. On the contrary, magnetic fields generated solely from a convective-core dynamo on the main sequence and detectable on the red giant branch would be buried too deep in the star and would not match the breakout timescales or the field strengths of magnetic white dwarfs. Therefore, for us to connect magnetic fields observed along the late evolution of stars via a fossil field we would need to find a broadly magnetized internal radiative zone on the red giant branch.

Les missions spatiales passées CoRoT et Kepler, actuellement en vol TESS, et en préparation PLATO permettent l'observation de millions d'étoiles. Dans cette thèse, nous nous intéressons en particulier aux géantes rouges (étoiles de type solaire évoluées). Grâce aux missions spatiales précédemment citées, il a été observé que le cœur de ces étoiles tourne environ dix fois plus rapidement que leur surface. Cependant, les modèles numériques d'évolution stellaire actuels prédisent une rotation beaucoup plus rapide: le cœur devrait tourner environ cent fois plus vite que la surface de l'étoile. Nous devons donc identifier le ou les mécanismes permettant d'évacuer efficacement le moment cinétique du cœur des étoiles de type solaire évoluées, une problématique qui est partagée par les étoiles de tout type pour tous les stades évolutifs. Dans ce contexte, des mesures très précises des paramètres stellaires globaux (comme la gravité de surface ou la masse) sont nécessaires pour contraindre au mieux les modèles d'évolution stellaires. L'étude des modes propres d'oscillation des étoiles (astérosismologie) joue un rôle clé car cette méthode permet l'estimation des paramètres stellaires avec une excellente précision. En revanche, cette précision n'est obtenue que lorsque les données sont peu bruitées et dans lesquelles les signaux des oscillations des étoiles sont bien visibles. En combinant les techniques classiques d'analyse photométrique et des algorithmes d'intelligence artificielle, nous obtenons dans le premier volet de cette thèse une meilleure estimation de la gravité de surface des étoiles de type solaire. Nous apportons en particulier des estimations précises de la gravité de surface lorsque l'astérosismologie standard atteint ses limites. Les étoiles de type solaire sont extraites de l'ensemble des données du satellite Kepler par un premier algorithme d'intelligence artificielle, et caractérisées par un second. Nous obtenons finalement les valeurs de gravité de surface des étoiles de type solaire avec une précision de 0.04 à 0.1 dex, entre la séquence principale (fusion de l'hydrogène dans le cœur) et les phases de géantes (fusion de l'hydrogène en couche et/ou de l'hélium dans le cœur), et ce directement à partir de la densité de puissance lumineuse moyenne émise par l'étoile. Ces étoiles bien caractérisées peuvent ensuite être étudiées plus en détail pour améliorer notre compréhension du transport de moment cinétique interne dans les étoiles de type solaire le long de leur évolution. Nous recherchons ensuite d'éventuelles signatures du phénomène de transport de moment cinétique actif dans les intérieurs stellaires dans les fréquences d'oscillation des étoiles de type solaire évoluées. Les champs magnétiques font partie des candidats les plus prometteurs pour expliquer le transport efficace observé. Les étoiles de masse intermédiaire (entre 1.1 et 3 masses solaires) sont connues pour développer un cœur convectif durant la séquence principale, qui peut générer un champ magnétique dynamo, relaxant dans l'intérieur radiatif sous forme d'un champ fossile et persistant durant le reste de l'évolution de l'étoile. Un tel champ magnétique devrait affecter les fréquences des modes d'oscillation mixtes acoustiques et de gravité des étoiles de masse intermédiaire évoluées étudiées dans cette thèse. Nous développons théoriquement la signature d'une telle configuration réaliste de champ magnétique fossile sur les fréquences des oscillations, en fonction de la masse, de la métallicité et de l'âge de l'étoile. Nous dérivons également une étude asymptotique complète, rendant possible l'inversion des données astérosismiques pour l'estimation des amplitudes des champs magnétiques internes. Nous estimons finalement l'efficacité d'un tel champ magnétique pour transporter le moment cinétique à l'intérieur des étoiles de type solaire.

The recent detection of solar equatorial Rossby waves has renewed interest in the study of gravito-inertial waves propagating in the convective envelope of solar-type stars. In particular, the ability of these envelope gravito-inertial modes to couple with those trapped in the radiative interior could open up new opportunities for probing the deep-layer dynamics of solar-type stars. The possibility for such a coupling to occur is particularly favoured among pre-main-sequence (PMS) solar-type stars. Indeed, due to the contraction of the protostellar object, they are able to reach high rotation frequencies before nuclear reactions are ignited and magnetic braking becomes the driving mechanism for their rotational evolution. In this work, we studied the coupling between the envelope inertial waves and the radiative interior g modes in PMS stars, focussing on the case of prograde dipolar modes. We considered the cases of 0.5 M _⊙ and 1 M _⊙ PMS models, each with three different scenarios of rotational evolution. We show that for stars that have formed with a sufficient amount of angular momentum, this coupling can occur in frequency ranges that are accessible to space-borne photometry, creating inertial dips in the period spacing pattern. Using an asymptotic analysis, we characterised the shape of these inertial dips to show that they depend on rotation and on the stiffness of the convective-radiative interface.

Dipolar ( ℓ = 1) mixed modes have revealed a surprisingly weak differential rotation between the core and the envelope of evolved solar-like stars. Quadrupolar ( ℓ = 2) mixed modes also contain information regarding internal dynamics but are very rarely characterised due to their low amplitude and the challenging identification of adjacent or overlapping rotationally split multiplets affected by near-degeneracy effects. We aim to extend the broadly used asymptotic seismic diagnostics beyond ℓ = 1 mixed modes by developing an analogue asymptotic description of ℓ = 2 mixed modes while explicitly accounting for near-degeneracy effects that distort their rotational multiplets. We have derived a new asymptotic formulation of near-degenerate mixed ℓ = 2 modes that describes off-diagonal terms representing the interaction between modes of adjacent radial orders. This formalism, expressed directly in the mixed-mode basis, provides analytical expressions for the near-degeneracy effects. We implemented the formalism within a global Bayesian mode-fitting framework for a direct fit of all ℓ = 0, 1, 2 modes in the power spectrum density. We were able to asymptotically model the asymmetric rotational splitting present in various radial orders of ℓ = 2 modes observed in young red giant stars without the need for any numerical stellar modelling. We applied our formalism to the Kepler target KIC 7341231, and it yielded core and envelope rotation rates consistent with previous numerical modelling while providing improved constraints from the global and model-independent approach. We also characterised the new target, KIC 8179973, measuring its rotation rate and mixed-mode parameters for the first time. As our framework relies on a direct global fit, it allows for much better precision on the asteroseismic parameters and rotation rate estimates than standard methods, yielding better constraints for rotation inversions. We have placed the first observational constraints on the asymptotic ℓ = 2 mixed-mode parameters (ΔΠ ₂ , q ₂ , and ε _g, 2 ), thus paving the way towards the use of asymptotic seismology beyond ℓ = 1 mixed modes.

Context. Asymmetries in the observed rotational splittings of a multiplet contain information about the star’s rotation profile and internal magnetic field. Moreover, the frequency regularities of multiplets can be used for mode identification. However, to exploit this information, highly accurate theoretical predictions are needed. Aims. We aim to quantify the difference in the predicted mode asymmetries between a 1D perturbative method and a 2D method that includes a 2D stellar structure model, which takes rotation into account. We then place these differences between 1D and 2D methods in the context of asteroseismic measurements of internal magnetic fields. We only focus on the asymmetries and not on possible additional frequency peaks that can arise when the magnetic and rotation axis are misaligned. Methods. We coupled the 1D pulsation codes GYRE and StORM to the 2D stellar structure code ESTER and compared the oscillation predictions with the results from the 2D TOP pulsation code. We focused on zero-age main-sequence models representative of rotating β Cephei pulsators spinning at up to 20 per cent of the critical Keplerian rotation rate. Specifically, we investigated low-radial-order gravity and pressure modes. Results. We find a generally good agreement between the oscillation frequencies resulting from the 1D and 2D pulsation codes. We report differences in predicted mode multiplet asymmetries of mostly below 0.06 d ⁻¹ . Since the magnetic asymmetries are small compared to the differences in the rotational asymmetries resulting from the 1D and 2D predictions, accurate measurements of the magnetic field are in most cases challenging. Conclusions. Differences in the predicted mode asymmetries of a rotating star between 1D perturbative methods and 2D non-perturbative methods can greatly hinder accurate measurements of internal magnetic fields in main-sequence pulsators with low-order modes. Nevertheless, reasonably accurate measurements could be possible with n _pg ≥ 2 modes if the internal rotation is roughly below 10 per cent of the Keplerian critical rotation frequency for (aligned) magnetic fields of the order of a few hundred kilogauss. While the differences between the 1D and 2D frequency predictions are mostly too large for internal magnetic field detections, the rotational asymmetries predicted by StORM are in general accurate enough for asteroseismic modelling of the stellar rotation in main-sequence stars with identified low-order modes.

Context. Rotation plays an important role in stellar evolution. However, the mechanisms behind the transport of angular momentum in stars at various stages of their evolution are not well understood. To improve our understanding of these processes, it is necessary to measure and validate the internal rotation profiles of stars across different stages of evolution and mass regimes. Aims. Our aim is to constrain the internal rotation profile of the 12-M _⊙ β Cep pulsator HD 192575 from the observed pulsational multiplets and the asymmetries of their component frequencies. Methods. We updated the forward asteroseismic modelling of HD 192575 based on new TESS observations. We inverted the rotation profile from the symmetric part of the splittings and computed the multiplet asymmetries due to the Coriolis force and stellar deformation, which we treated perturbatively. We compared the computed asymmetries with the observed asymmetries. Results. Our new forward asteroseismic modelling is in agreement with previous results but with increased uncertainties, partially due to increased frequency precision, which required us to relax certain constraints. Ambiguity in the mode identification is the main source of the uncertainty, which also affects the inferred rotation profiles. Almost all acceptable rotation profiles occur in the regime below 0.4 d ⁻¹ and favour weak radial differential rotation, with a ratio of core to envelope rotation of less than 2. We find that the quality of the match between the observed and theoretically predicted mode asymmetries is strongly dependent on the mode identification and the internal structure of the star. Conclusions. Our results offer the first detailed rotation inversion for a β Cep pulsator. They show that the rotation profile and the mode asymmetries provide a valuable tool for further constraining the evolutionary properties of HD 192575, and in particular the details of angular momentum transport in massive stars.

Context. Recent observational results from asteroseismic studies show that an important fraction of solar-like stars do not present detectable stochastically excited acoustic oscillations. This non-detectability seems to correlate with a high rotation rate in the convective envelope and a high surface magnetic activity. At the same time, the properties of stellar convection are affected by rotation and magnetism. Aims. We investigate the role of rotation in the excitation of acoustic modes in the convective envelope of solar-like stars, to evaluate its impact on the energy injected in the oscillations. Methods. We derived theoretical prescriptions for the excitation of acoustic waves in the convective envelope of rotating solar-like stars. We adopted the rotating mixing-length Theory to model the influence of rotation on convection. We used the MESA stellar evolution code and the GYRE stellar oscillation code to estimate the power injected in the oscillations from our theoretical prescriptions. Results. We demonstrate that the power injected in the acoustic modes is insensitive to rotation if a Gaussian time-correlation function is assumed, while it can decrease by up to 60% for a Lorentzian time-correlation function, for a 20 Ω_⊙ rotation rate. We show that the modification of the excitation rate by rotation depends not only on the rotation rate but also on the radial and angular orders of the considered oscillation mode. This result can allow for better constraints on the properties of stellar convection by studying observationally acoustic mode excitation. Conclusions. These results demonstrate how important it is to take into account the modification of stellar convection by rotation when evaluating the amplitude of the stellar oscillations it stochastically excites. They open the path for understanding the large variety of observed acoustic-mode amplitudes at the surface of solar-like stars as a function of surface rotation rates.

Gamma-Dor stars are ideal targets for studies of stellar innermost dynamical properties due to their rich asteroseismic spectrum of gravity modes. Integrating internal magnetism to the picture appears as the next milestone of detailed asteroseismic studies, for its prime importance on stellar evolution. The inertial dip in prograde dipole modes period-spacing pattern of gamma-Dors stands out as a unique window on the convective core structure and dynamics. Recent studies have highlighted the dependence of the dip structure on core density stratification, contrast of the near-core Brunt-Väisälä frequency and rotation rate, as well as the core-to-near-core differential rotation. In the meantime, the effect of magnetism has been derived on envelope modes. We aim to revisit the inertial dip formation including core and envelope magnetism, and explore the probing power of this feature on dynamo-generated core fields. We consider a toroidal magnetic field with a bi-layer (core and envelope) Alfvén frequency. This configuration allows us to revisit the coupling problem using our knowledge on both core magneto-inertial modes and envelope magneto-gravito-inertial modes. We stay in an analytical framework to exhibit the magnetic effects on the inertial dip shape and location, setting up a laboratory towards the comprehension of magnetic effects on the dip structure. We show a shift of the inertial dip towards lower spin parameter values and a thinner dip with increasing core magnetic field, quite similar to the signature of differential rotation. The magnetic effects become sizeable when the ratio between the magnetic and the Coriolis effects is large enough. We explore the degeneracy of the magnetic effects with differential rotation. We study the detectability of core magnetism, considering observational constraints on the modes periods and potential gravito-inertial mode suppression.