Low mass stars seismology with WhoSGlAd and EGGMiMoSA

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.Thèse de doctorat : New seismic probing method for solar-type stars, red subgiants and g pulsator

Good Vibrations: Sismologie d'étoiles de masse faible avec WhoSGlAd et EGGMiMoSA

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.Thèse de doctorat : New seismic probing method for solar-type stars, red subgiants and g pulsator

Caractérisation exhaustive du système 16 Cygni. I. Modélisation "Forward" avec WhoSGlAd

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.Thèse de doctorat : New seismic probing method for solar-type stars, red subgiants and g pulsator

Climatic and cultural changes in the west Congo Basin forests over the past 5000 years

Central Africa includes the world's second largest rainforest block. The ecology of the region remains poorly understood, as does its vegetation and archaeological history. However, over the past 20 years, multidisciplinary scientific programmes have enhanced knowledge of old human presence and palaeoenvironments in the forestry block of Central Africa. This first regional synthesis documents significant cultural changes over the past five millennia and describes how they are linked to climate. It is now well documented that climatic conditions in the African tropics underwent significant changes throughout this period and here we demonstrate that corresponding shifts in human demography have had a strong influence on the forests. The most influential event was the decline of the strong African monsoon in the Late Holocene, resulting in serious disturbance of the forest block around 3500 BP. During the same period, populations from the north settled in the forest zone; they mastered new technologies such as pottery and fabrication of polished stone tools, and seem to have practised agriculture. The opening up of forests from 2500 BP favoured the arrival of metallurgist populations that impacted the forest. During this long period (2500–1400 BP), a remarkable increase of archaeological sites is an indication of a demographic explosion of metallurgist populations. Paradoxically, we have found evidence of pearl millet (Pennisetum glaucum) cultivation in the forest around 2200 BP, implying a more arid context. While Early Iron Age sites (prior to 1400 BP) and recent pre-colonial sites (two to eight centuries BP) are abundant, the period between 1600 and 1000 BP is characterized by a sharp decrease in human settlements, with a population crash between 1300 and 1000 BP over a large part of Central Africa. It is only in the eleventh century that new populations of metallurgists settled into the forest block. In this paper, we analyse the spatial and temporal distribution of 328 archaeological sites that have been reliably radiocarbon dated. The results allow us to piece together changes in the relationships between human populations and the environments in which they lived. On this basis, we discuss interactions between humans, climate and vegetation during the past five millennia and the implications of the absence of people from the landscape over three centuries. We go on to discuss modern vegetation patterns and African forest conservation in the light of these events.Peer reviewe

On the angular momentum transport by internal gravity waves at the time of asteroseismology

Les missions spatiales CoRoT (2006-2014) et Kepler (2009) ont procuré de nombreuses données sismiques pour des milliers d'étoiles de faible masse. L'analyse de ces données a rendu possible l'étude de l'intérieur de ces étoiles au cours de l'évolution et a apporté de fortes contraintes sur les processus physiques à l’œuvre sous leur surface. En particulier, ces observations ont montré que la rotation moyenne du cœur de ces étoiles augmente légèrement avec le temps sur la branche des sous-géantes avant de diminuer lors de l'ascension de la branche des géantes rouges. Ceci est désaccord avec les prédictions théoriques actuelles et souligne la nécessité d'inclure de nouveaux processus de transport de moment cinétique dans les modèles stellaires. Dans une première partie, j'ai donc étudié l'influence du transport de moment cinétique par les ondes internes de gravité sur l'évolution de la rotation dans les étoiles de faible masse. Ces ondes se propagent dans les zones radiatives stablement stratifiées et sont capables d'en modifier la vitesse de rotation moyenne. Or, l'efficacité du transport par les ondes dépend de leur amplitude et donc du mécanisme d'excitation. Alors que des modèles semi-analytiques permettaient déjà d'évaluer l'énergie transférée aux ondes par la pression turbulente dans la zone convective, une estimation théorique de l'excitation par la pénétration de panaches convectifs à l'interface avec la zone radiative restait manquante. J'ai donc proposé un modèle d'excitation pour estimer la part d'énergie cinétique des panaches transférées sous forme d'ondes à la base de la zone convective (Pinçon et al., 2016). Cela m'a d'abord permis d'établir que la pénétration convective génère des ondes plus efficacement que la pression turbulente, et ensuite que les ondes induites par la pénétration convective sont capables de s'opposer à l'accélération de la rotation due à la contraction des couches internes, depuis la séquence principale jusqu'au début de l'ascension de la branche des géantes rouges. En particulier, j'ai montré que les valeurs de la rotation observées dans l'intérieur des étoiles sous-géantes peuvent être interprétées comme le possible résultat d'un mécanisme de régulation contrôlé par ces ondes (Pinçon at al., 2017). Dans une seconde partie, je me suis intéressé à l'amélioration et à l'élaboration des diagnostiques sismiques par les modes mixtes, ces modes d'oscillation qui sont capables de sonder aussi bien l'enveloppe que les régions centrales des étoiles. Les diagnostiques sismiques font le lien entre les caractéristiques observées dans un spectre d'oscillation et les propriétés de la structure interne de l'étoile. Mon attention s'est premièrement focalisée sur la facteur de couplage des modes mixtes qui décrit le degré d'interaction entre les oscillations dans la cavité centrale et celles dans l'enveloppe de l'étoile. Ce paramètre n'a été, jusque là, que très peu étudié. Une première étude observationnelle sur un large échantillon d'étoiles par Mosser et al. (2017) a montré que ce facteur varie au cours de l'évolution et se comporte différemment selon le stade évolutif. J'ai contribué à l'interprétation des résultats en montrant via un modèle simplifié que ce facteur est sensible aux changements structuraux de l'étoile au cours de l'évolution. De plus, cette analyse a notamment démontré la nécessité de considérer l'hypothèse d'un fort couplage. J'ai donc entrepris une validation du formalisme proposé parallèlement à cette dernière étude par Takata (2016) en comparant ses prédictions avec celles obtenues numériquement pour des modèles d'étoiles évoluées. Enfin, en utilisant une modélisation simple, j'ai montré qu'une analyse précise du spectre des modes mixtes paramètre permettrait de plus d'obtenir de l'information sur le contraste de densité entre le coeur et l'enveloppe de l'étoile.The space-borne missions CoRoT (2006-2014) and Kepler (2009) provided a lot of seismic data for thousands of low-mass stars. Data analysis enabled us to study the interior of these stars during their evolution and brought stringent constraints on the physical processes at work under their surface.These observations notably revealed that the mean core rotation rate of stars weakly increases on the subgiant branch before dropping on the red giant branch while their central layers are contracting.for several subgiant and red giant stars in which mixed modes could be detected. Subsequently, several works demonstrated the inability of the current stellar evolution codes to reproduce these observations and stressed out the need for an additional transport process of angular momentum to counteract the acceleration of the central rotation driven by the core contraction during the post-main sequence evolution.Therefore, in a first part of my PhD thesis, I investigated the effect of the angular momentum transport by internal gravity waves on the rotation evolution of low-mass stars. These waves have buoyancy as restoring force and can propagate into stably stratified radiative zones, where they are able to interact with the medium and modify its mean rotation. The efficiency of the angular momentum transport by waves depends on their amplitude and so on their generation mechanism. While several works had already theoretically studied the wave excitation by turbulent pressure in the convective, an estimate of the wave generation by penetrative convection into the upper layers of the radiatve zone was still missing. I thus developed a semi-analytical excitation model to estimate the part of the plumes kinetic energy transferred into internal gravity waves at the base of the convective zone (Pinçon et al., 2016). I first found that penetrative convection generates waves more efficiently than turbulent pressure, and then that plume-induced waves are able to counteract the spin-up of the core driven by the gravitational contraction from the main-sequence to the beginning of the ascent of the red giant branch. Moreover, I showed that the radial-differential rotation observed in subgiant and early red giant stars can be explained by a regulation mechanism controlled by the influence of the plume-induced internal gravity waves (Pinçon et al., 2017).In a second part, I worked on the elaboration and the improvement of the seismic diagnoses by mixed modes that have the ability to probe both the envelope and the core of stars. Seismic diagnoses aim at making the link between the features observed in oscillation spectra and the physical quantities describing stars and their internal structures. In a first step, I focused on the coupling factor of mixed modes which expresses the level of interaction between the central and the outer resonant cavities and had still remained largely unexploited. The first large-scale analysis of this parameter by Mosser et al. (2017) showed that this factor vary during the evolution, with typical values depending on the evolutionary status.In this work, I contributed to the interpretation of the results via a simplified model in which the value of the coupling factor is directly sensitive to structural readjustments occurring during stellar evolution. This study notably revealed the necessity to consider the hypothesis of a strong coupling. In parallel to this work, a theoretical description of mixed modes under the assumption of strong coupling was proposed by Takata (2016). Therefore, I undertook a validation of this formalism by comparing its predictions with an oscillation code for appropriate evolved models. Finally, using a simplifying modeling, I showed that a precise analysis of the mixed modes spectrum can also bring information on the contrast of density between the core and the envelope

Du transport de moment cinétique par les ondes internes de gravité à l'heure de la sismologie stellaire

The space-borne missions CoRoT (2006-2014) and Kepler (2009) provided a lot of seismic data for thousands of low-mass stars. Data analysis enabled us to study the interior of these stars during their evolution and brought stringent constraints on the physical processes at work under their surface.These observations notably revealed that the mean core rotation rate of stars weakly increases on the subgiant branch before dropping on the red giant branch while their central layers are contracting.for several subgiant and red giant stars in which mixed modes could be detected. Subsequently, several works demonstrated the inability of the current stellar evolution codes to reproduce these observations and stressed out the need for an additional transport process of angular momentum to counteract the acceleration of the central rotation driven by the core contraction during the post-main sequence evolution.Therefore, in a first part of my PhD thesis, I investigated the effect of the angular momentum transport by internal gravity waves on the rotation evolution of low-mass stars. These waves have buoyancy as restoring force and can propagate into stably stratified radiative zones, where they are able to interact with the medium and modify its mean rotation. The efficiency of the angular momentum transport by waves depends on their amplitude and so on their generation mechanism. While several works had already theoretically studied the wave excitation by turbulent pressure in the convective, an estimate of the wave generation by penetrative convection into the upper layers of the radiatve zone was still missing. I thus developed a semi-analytical excitation model to estimate the part of the plumes kinetic energy transferred into internal gravity waves at the base of the convective zone (Pinçon et al., 2016). I first found that penetrative convection generates waves more efficiently than turbulent pressure, and then that plume-induced waves are able to counteract the spin-up of the core driven by the gravitational contraction from the main-sequence to the beginning of the ascent of the red giant branch. Moreover, I showed that the radial-differential rotation observed in subgiant and early red giant stars can be explained by a regulation mechanism controlled by the influence of the plume-induced internal gravity waves (Pinçon et al., 2017).In a second part, I worked on the elaboration and the improvement of the seismic diagnoses by mixed modes that have the ability to probe both the envelope and the core of stars. Seismic diagnoses aim at making the link between the features observed in oscillation spectra and the physical quantities describing stars and their internal structures. In a first step, I focused on the coupling factor of mixed modes which expresses the level of interaction between the central and the outer resonant cavities and had still remained largely unexploited. The first large-scale analysis of this parameter by Mosser et al. (2017) showed that this factor vary during the evolution, with typical values depending on the evolutionary status.In this work, I contributed to the interpretation of the results via a simplified model in which the value of the coupling factor is directly sensitive to structural readjustments occurring during stellar evolution. This study notably revealed the necessity to consider the hypothesis of a strong coupling. In parallel to this work, a theoretical description of mixed modes under the assumption of strong coupling was proposed by Takata (2016). Therefore, I undertook a validation of this formalism by comparing its predictions with an oscillation code for appropriate evolved models. Finally, using a simplifying modeling, I showed that a precise analysis of the mixed modes spectrum can also bring information on the contrast of density between the core and the envelope.Les missions spatiales CoRoT (2006-2014) et Kepler (2009) ont procuré de nombreuses données sismiques pour des milliers d'étoiles de faible masse. L'analyse de ces données a rendu possible l'étude de l'intérieur de ces étoiles au cours de l'évolution et a apporté de fortes contraintes sur les processus physiques à l’œuvre sous leur surface. En particulier, ces observations ont montré que la rotation moyenne du cœur de ces étoiles augmente légèrement avec le temps sur la branche des sous-géantes avant de diminuer lors de l'ascension de la branche des géantes rouges. Ceci est désaccord avec les prédictions théoriques actuelles et souligne la nécessité d'inclure de nouveaux processus de transport de moment cinétique dans les modèles stellaires. Dans une première partie, j'ai donc étudié l'influence du transport de moment cinétique par les ondes internes de gravité sur l'évolution de la rotation dans les étoiles de faible masse. Ces ondes se propagent dans les zones radiatives stablement stratifiées et sont capables d'en modifier la vitesse de rotation moyenne. Or, l'efficacité du transport par les ondes dépend de leur amplitude et donc du mécanisme d'excitation. Alors que des modèles semi-analytiques permettaient déjà d'évaluer l'énergie transférée aux ondes par la pression turbulente dans la zone convective, une estimation théorique de l'excitation par la pénétration de panaches convectifs à l'interface avec la zone radiative restait manquante. J'ai donc proposé un modèle d'excitation pour estimer la part d'énergie cinétique des panaches transférées sous forme d'ondes à la base de la zone convective (Pinçon et al., 2016). Cela m'a d'abord permis d'établir que la pénétration convective génère des ondes plus efficacement que la pression turbulente, et ensuite que les ondes induites par la pénétration convective sont capables de s'opposer à l'accélération de la rotation due à la contraction des couches internes, depuis la séquence principale jusqu'au début de l'ascension de la branche des géantes rouges. En particulier, j'ai montré que les valeurs de la rotation observées dans l'intérieur des étoiles sous-géantes peuvent être interprétées comme le possible résultat d'un mécanisme de régulation contrôlé par ces ondes (Pinçon at al., 2017). Dans une seconde partie, je me suis intéressé à l'amélioration et à l'élaboration des diagnostiques sismiques par les modes mixtes, ces modes d'oscillation qui sont capables de sonder aussi bien l'enveloppe que les régions centrales des étoiles. Les diagnostiques sismiques font le lien entre les caractéristiques observées dans un spectre d'oscillation et les propriétés de la structure interne de l'étoile. Mon attention s'est premièrement focalisée sur la facteur de couplage des modes mixtes qui décrit le degré d'interaction entre les oscillations dans la cavité centrale et celles dans l'enveloppe de l'étoile. Ce paramètre n'a été, jusque là, que très peu étudié. Une première étude observationnelle sur un large échantillon d'étoiles par Mosser et al. (2017) a montré que ce facteur varie au cours de l'évolution et se comporte différemment selon le stade évolutif. J'ai contribué à l'interprétation des résultats en montrant via un modèle simplifié que ce facteur est sensible aux changements structuraux de l'étoile au cours de l'évolution. De plus, cette analyse a notamment démontré la nécessité de considérer l'hypothèse d'un fort couplage. J'ai donc entrepris une validation du formalisme proposé parallèlement à cette dernière étude par Takata (2016) en comparant ses prédictions avec celles obtenues numériquement pour des modèles d'étoiles évoluées. Enfin, en utilisant une modélisation simple, j'ai montré qu'une analyse précise du spectre des modes mixtes paramètre permettrait de plus d'obtenir de l'information sur le contraste de densité entre le coeur et l'enveloppe de l'étoile

Probing the mid-layer structure of red giants

27 pages, accepted in A\&AInternational audienceContext. The space-borne missions CoRoT and Kepler have already brought stringent constraints on the internal structure of low-mass evolved stars, a large part of which results from the detection of mixed modes. However, all the potential of these oscillation modes as a diagnosis of the stellar interior has not been fully exploited yet. In particular, the coupling factor or the gravity-offset of mixed modes, q and ɛg, are expected to provide additional constraints on the mid-layers of red giants, which are located between the hydrogen-burning shell and the neighborhood of the base of the convective zone. The link between these parameters and the properties of this region, nevertheless, still remains to be precisely established.Aims: In the present paper, we investigate the potential of the coupling factor in probing the mid-layer structure of evolved stars.Methods: Guided by typical stellar models and general physical considerations, we modeled the coupling region along with evolution. We subsequently obtained an analytical expression of q based on the asymptotic theory of mixed modes and compared it to observations.Results: We show that the value of q is degenerate with respect to the thickness of the coupling evanescent region and the local density scale height. On the subgiant branch and the beginning of the red giant branch (RGB), the model predicts that the peak in the observed value of q is necessarily associated with the important shrinking and the subsequent thickening of the coupling region, which is located in the radiative zone at these stages. The large spread in the measurement is interpreted as the result of the high sensitivity of q to the structure properties when the coupling region becomes very thin. Nevertheless, the important degeneracy of q in this regime prevents us from unambiguously concluding on the precise structural origin of the observed values. In later stages, the progressive migration of the coupling region toward the convective zone is expected to result in a slight and smooth decrease in q, which is in agreement with observations. At one point just before the end of the first-dredge up and the luminosity bump, the coupling region becomes entirely located in the convective region and its continuous thickening is shown to be responsible for the observed decrease in q. We demonstrate that q has the promising potential to probe the migration of the base of the convective region as well as convective extra-mixing during this stage. We also show that the frequency-dependence of q cannot be neglected in the oscillation spectra of such evolved RGB stars, which is in contrast with what is assumed in the current measurement methods. This fact can have an influence on the physical interpretation of the observed values. In red clump stars, in which the coupling regions are very thin and located in the radiative zone, the small variations and spread observed in q suggest that their mid-layer structure is very stable.Conclusions: A structural interpretation of the global observed variations in q was obtained and the potential of this parameter in probing the dynamics of the mid-layer properties of red giants is highlighted. This analytical study paves the way for a more quantitative exploration of the link of q with the internal properties of evolved stars using stellar models for a proper interpretation of the observations. This will be undertaken in the following papers of this series

Implications of the generation of internal gravity waves by penetrative convection for the internal rotation evolution of low-mass stars

Due to the space-borne missions CoRoT and Kepler, noteworthy breakthroughs have been made in our understanding of stellar evolution, and in particular about the angular momentum redistribution in stellar interiors. Indeed, the high-precision seismic data provide with the measurement of the mean core rotation rate for thousands of low-mass stars from the subgiant branch to the red giant branch. All these observations exhibit much lower core rotation rates than expected by current stellar evolution codes and they emphasize the need for an additional transport process. In this framework, internal gravity waves (herefater, IGW) could play a signifivative role since they are known to be able to transport angular momentum. In this work, we estimate the effciency of the transport by the IGW that are generated by penetrative convection at the interface between the convective and the radiative regions. As a first step, this study is based on the comparison between the timescale for the waves to modify a given rotation profile and the contraction/expansion timescale throughout the radiative zone of 1.3M⊙ stellar models. We show that IGW, on their own, are ineffcient to slow down the core rotation of stars on the red giant branch, where the radiative damping becomes strong enough and prevent the IGW from reaching the innermost layers. However, we find that IGW generated by penetrative convection could effciently modify the core rotation of subgiant stars as soon as the amplitude of the radial differential rotation between the core and the base of the convective zone is high enough, with typical values close to the observed rotation rates in these stars. This result argues for the necessity to account for IGW generated by penetrative convection in stellar modeling and in the angular momentum redistribution issue

Influence of plume-induced internal gravity waves on the rotation profile of low-mass stars

International audienc

Implications of the generation of internal gravity waves by penetrative convection for the internal rotation evolution of low-mass stars

International audienc