The recent space-based exoplanet transit searches, CoRoT and Kepler, have revolutionised the field of stellar variability. In this thesis I exploit the public Kepler data to characterise stellar variability, and study rotation periods. For the study of stellar variability it is a complicated but necessary process to remove instrumental systematics while maintaining intrinsic stellar signal. I was involved in the development of a new correction method for systematics, denoted ARC (Astrophysically Robust Correction). This method relies on the removal of a set of basis functions that are determined to be present in small amounts across many light curves. Using the first month of Kepler data, corrected with the ARC method, I studied the variability properties of main sequence stars as a function of fundamental stellar parameters. I find that the fraction of stars with variability greater than that of the Sun is 60%, and confirm the trend of increasing variability with decreasing effective temperatures. I show tentative evidence that the more active stars have lower proper motions and may be located closer to the galactic plane. I also investigate the frequency content of the variability, showing that there exist significant differences in the nature of variability between spectral types, with a trend towards longer periods at later spectral types. In order to exploit the full potential of the Kepler data for stellar rotation period measurement, I developed a novel method of period detection for use on star spot modulated light curves. Standard approaches to period detection are based on Fourier decomposition or least-squares fitting of sinusoidal models. However, typical stellar light curves are neither sinusoidal nor strictly periodic. Therefore, I developed an algorithm for period detection based on the autocorrelation function (ACF) of the light curve. Because the ACF measures only the degree of self-similarity of the light curve at a given time lag, the period remains detectable even when the amplitude and phase of the photometric modulation evolve significantly. I applied the ACF method for the sample of M-dwarfs observed during the first 10 months of the Kepler mission, and detected rotation periods in 1570, ranging from 0.37-69.7 days. The rotation period distribution is clearly bimodal, with peaks at ~19 and ~33 days, hinting at two distinct waves of star formation. These two peaks form two distinct sequences in period-temperature space, with the period decreasing with increasing temperature. In a natural continuation to this work I applied measured periods for 1000 stars in each of the F, G and K-dwarf sets observed by Kepler, and combined these with the M-dwarf results. The trend of increasing rotation period with increasing mass is clear throughout, as the observations fall along a wide by distinct sequence. Comparison to the rotational isochrones of Barnes (2007) show an overall agreement, although the dataset, which I believe is the largest set of rotation period measurements for main sequence stars, shows addition detail, not captured by the gyrochronology relations. This includes a dip in the rotation period distribution at ~0.6 M⊙ and a steep increase in period for the M-dwarfs. I also applied the ACF method to the Kepler exoplanet candidate host stars and used the results to search for evidence of tidal interaction between the star and planet. I show that for the majority of exoplanet host stars, spin-orbit interaction will not have affected the stellar rotation period, permitting the application of gyrochronology for age determination. A comparison of the host stars with a sample of field stars selected to match their temperature and magnitude distribution also indicates no significant difference in the period or amplitude distributions of the two sets. The only notable variation is the lack of planets around the very fast rotators across all spectral types.

Le satellite spatial Kepler a détecté des oscillations de type solaire parmi plusieurs centaines d'étoiles, permettant la détermination de leurs propriétés physiques à l'aide de l’astérosismologie. Les modèles d'évolution stellaire et les lois d'échelle employés pour déterminer les paramètres tels que la masse, le rayon et l'âge nécessitent toutefois une calibration adaptée. Dans ce contexte, l'utilisation des systèmes binaires présentant des oscillations de type solaires pour les deux étoiles semble particulièrement appropriée. Au cours de cette thèse, nous avons procédé à un travail de détection de ces binaires sismiques parmi les données de Kepler ainsi qu'au développement des outils nécessaires à leur analyse. Bien que la découverte d'une nouvelle binaire sismique semblait très peu probable, nous avons pu rapporter pour la toute première fois la détection d'oscillations de type solaire associées aux deux étoiles les plus brillantes d'un système triple, à savoir HD 188753. À partir de la modélisation, nous avons déterminé des âges semblables pour les deux étoiles détectées en astérosismologie, comme attendu en raison de leur origine commune. Par ailleurs, nous avons entrepris la première analyse orbitale de ce système hiérarchique dans le but d'obtenir une estimation directe des masses et de la parallaxe. Finalement, l'exemple de HD 188753 illustre notre capacité à détecter et à modéliser chacune des étoiles d'un système binaire ou multiple tout en réalisant l'analyse orbitale de celui-ci. Les différents outils développés au cours de cette thèse seront intensivement utilisés dans le cadre des futures missions TESS et PLATO. / The Kepler space telescope detected solar-like oscillations in several hundreds of stars, providing a way to determine their physical properties using asteroseismology. However, the stellar evolutionary models and scaling relations employed to determine parameters such as the mass, the radius and the age require a proper calibration. In this context, the use of seismic binaries showing solar-like oscillations in both stars is especially suitable. During this thesis, we have worked towards the detection of such seismic binaries from the Kepler database and developed the necessary tools to study them. Although the discovery of a new seismic binary was very unlikely, we were able to report for the first time the detection of solar-like oscillations in the two brightest stars of a triple stellar system, namely HD 188753. Using stellar modelling, we found compatible ages for the two stars derived from asteroseismology, as expected from their common origin. In addition, we performed the first orbital analysis of this hierarchical system in order to derive a direct estimate of masses and parallax. Finally, the example of HD 188753 shows our capability to detect and model each of the stars of a binary or multiple system and to perform the orbital analysis of this one. The various tools developed during this thesis will be extensively used in the context of the future missions TESS and PLATO.

Comprendre les variations de l’irradiance solaire, notamment dans le domaine UV, est essentiel pour les modèles climatiques. Les modèles d’irradiance sont précieux pour reconstruire l'irradiance solaire spectrale (SSI) en l'absence d'observations ou lorsque celles-ci manquent de stabilité. Cependant, ils font certaines hypothèses sur les structures solaires. Nous visons ici à contraindre ces hypothèses en caractérisant le contraste UV des structures solaires. Grâce aux données du satellite Solar Dynamic Observatory (SDO)entre 2010 et 2016, nous quantifions pour la première fois les contrastes dans l'UV. L'étude du contraste des structures solaires et de leur segmentation montre que des seuils photométriques sont nécessaires pour segmenter correctement les structures solaires, principalement en UV, en raison de la coexistence de structures sombres et brillantes pour la même valeur du champ magnétique. Certains pixels classés parmi le Soleil calme par le modèle SATIRE-S appartiennent en fait aux facules, mais ils sont trop peu nombreux pour avoir un impact sur les reconstructions de SSI. Nos résultats soulignent l'importance des observations multi-longueurs d'onde pour mieux contraindre l'identification des structures. Distinguer réseau et facule est essentiel pour reconstruire la SSI sur une longue période, et la prise en compte de la dépendance du contraste du réseau par rapport au champ magnétique améliore la reconstruction de la SSI. Enfin, nous ne trouvons aucun indice de variations du contraste durant le cycle solaire.Nous présentons aussi des résultats sur les corrélations entre les émissions Hα et Ca II des étoiles de type solaire. Nous montrons que l’hypothèse de Meunier et al., 2009 pour expliquer les anti-corrélations de certaines étoiles est confirmée par nos résultats. / Understanding solar irradiance variations,in particular in the ultraviolet wavelength range, is essential for climate modelling. Solar irradiance models are precious for reconstructing the spectral solar irradiance (SSI) in the absence of observations or when they lack stability. However, they come with their assumptions. Here we aim here to constrain these in the UV by characterising the contrast of solar magnetic features in the UV.From solar images taken by the Solar Dynamic Observatory (SDO) between 2010 and 2016, we quantify UV contrasts the first time. The study of the contrast of the solar structures and their segmentation shows that photometric thresholds are necessary to properly segment solar structures, mainly in the UV, because of the coexistence of both dark and bright structures for the same value of the magnetic field. Some pixels that are classified as quiet-Sun by the SATIRE-S model actually belong to faculae, but they are too few to have a significant impact on SSI reconstructions. Our results highlight the importance of multi-wavelength observations for better constraining the identification of structures. Distinguishing network and faculae is essential for such reconstructions over a long period, and using a network with magnetically variable contrast improves SSI reconstruction, Finally, we find no evidence of contrast variations during the solar cycle.We also present results on the correlations between Hα and Ca II emissions of Sun-like stars. We show that the hypothesis of Meunier et al., 2009 to explain the anti-correlations of certain stars corresponds to our observations.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第23011号 / 理博第4688号 / 新制||理||1672(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 浅井 歩, 教授 嶺重 慎, 教授 一本 潔 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Flows, instabilities, and magnetism play significant roles in the internal and atmospheric dynamics of objects ranging from the smallest exoplanets to the largest stars. These phenomena are governed by the equations of magnetohydrodynamics (MHD), which link the flows and magnetic fields, and from which the operational parameters and growth rates of instabilities can be recovered. Here we present an overview of interesting phenomena (such as the internal dynamics of stellar and planetary objects, as well as instabilities which might operate within these environs), as well as computational techniques by which these phenomena might both be understood and analysed (through both ‘simplifications’ of the MHD equations and different numerical/computational approaches). We first present an investigation into the Heat-Flux-Driven Buoyancy Instability (HBI) within stellar and planetary atmospheres, considering both the parameter space it might operate within as well as its non-linear effects during said operation. We find that whilst the HBI may be able to play a role in Solar, stellar and planetary atmospheres, it is likely to be quite limited in scope, only operating within small regions. However, its dramatic consequences for heat transport in the non-linearly evolved state, and the prospects that it may operate outside the narrow regimes that our analytical analysis suggested, suggest that it may merit further study. This is followed with a discussion of a method by which the surface flows of exoplanets might be measured: The Rossiter-Mclaughlin Effect at Secondary Eclipse (RMse). We formulate the effect, showing that the formalism is identical to the traditional Rossiter-Mclaughlin effect, albeit in a different frame (a planet transiting a star becomes a star transiting a planet), and consider its observational implications: the effect should be observable for the brightest planet hosting stars using upcoming 40m-class telescopes (i.e.E-ELT). We finish with a series of 3D anelastic simulations of fully convective stars, designed to investigate how the internal flows are affected by varying stellar parameters, as well as a possible link between residual entropy and differential rotation contours, and a method by which this link can be used (via the thermal wind equation - TWE) to extrapolate the internal rotation. We find a clear transition between ‘solar-like’ and ‘anti-solar’ internal dynamics, characterised in the meridional circulation, differential rotation, residual entropy, and angular momentum flux profiles. Furthermore we find that, whilst the alignment between residual entropy and differential rotation contours is somewhat varied, the resultant extrapolation, via the TWE, produces a generally good fit to the differential rotation contours, suggesting a general robustness to the theory.

No description available.

Dans le cadre de ma thèse, j'ai effectué l'analyse spectrale d'un échantillon massif de spectres stellaires dans le contexte du programme exoplanète de la mission CoRoT. J'ai tiré avantage des instruments Flames/GIRAFFE pour observer près de 2000 étoiles dans le but de comprendre les populations stellaires dans les champs CoRoT. Dans ce but, j'ai implémenté, calibré, testé, et appliqué une chaîne de traitement et de réduction fiable et efficace afin de réduire et d'analyser automatiquement (en utilisant l'algorithme de paramétrisation MATISSE, Gazzano et al. 2010) un large échantillon de spectres stellaires. J'ai déterminé la vitesse barycentrique radiale, une estimation de la vitesse de rotation projetée sur la ligne de visée, la température effective, la gravité de surface, de la métallicité global et l'enrichissement des éléments par rapport au fer pour 1 227 étoiles dans trois des champs CoRoT. Ainsi, j'ai construit un des premiers échantillons affranchis de biais de sélection pour toute étude concernant la relation planète métallicité dans les champs CoRoT et démontré que le nombre des étoiles naines a été généralement sous-estimé par la classification photométrique (Exo-Dat, Deleuil et al. 2009). J'ai appliqué la relation reliant le nombre de planètes détectées à la métallicité de l'étoile hôte (Udry & Santos 2007), parfaitement en accord avec le nombre actuel de détection planétaire dans les champs CoRoT correspondant (Gazzano et al. 2010). En utilisant les paramètres atmosphériques MATISSE, nous avons déterminé les distances et nous les avons combinées avec des informations cinématiques (les mouvements propres du catalogue PPMXL - Roeser et al. (2010), et l'astrométrie). Nous avons dérivé les composantes de cinématique Galactique : position et vitesse. Cela m'a permis d'étudier les populations stellaires dans les champs CoRoT /exoplanète et de quantifier le gradient de métallicité dans la Galaxie (Gazzano et al. En préparation). J'ai par ailleurs montré que les étoiles à planètes détectées dans les champs considérés pour ma thèse sont exclusivement des étoiles de disque mince. J'ai également participé au suivi des candidats planète CoRoT à l'aide de spectroscopie à haute résolution. J'ai effectué l'analyse spectrale, avec le logiciel VWA (Bruntt et al. 2010b,a), des étoiles hôtes pour la mission spatiale CoRoT. Ces études ont conduit à la détermination des paramètres fondamentaux de l'étoile, qui est une étape indispensable pour la caractérisation complète de la planète. / During my Ph.D., I performed the spectral analysis of a massive sample of stellar spectra in the context of the CoRoT /Exoplanet mission. We took advantage of the Flames/GIRAFFE multi-fibre instrument to observe almost 2 000 stars with the aim of understanding the stellar populations in the CoRoT fields. To these purposes, I implemented, calibrated, tested and applied an automatic pipeline to reduce and analyse automatically (using the parameterization algorithm MATISSE, Gazzano et al. 2010) a large sample of stellar spectra. I derived the barycentric radial velocity, an estimate of the rotational velocity projected on the line of sight, the effective temperature, the surface gravity, the overall metallicity and the -enhancement for 1227 stars in three of the CoRoT fields. Hence, I built one of the first unbiased samples for any study regarding planet metallicity relationship in the CoRoT fields and demonstrated that the amount of dwarf stars was generally underestimated by the photometric classification (in Exo-Dat, Deleuil et al. 2009). I applied the relationship linking the number of planets as a function of the metallicity of the host star (Udry & Santos 2007), totally in agreement with the current number of planetary detection in the corresponding CoRoT fields (Gazzano et al. 2010). Using MATISSE atmospheric parameters, we determined distances and combining them with kinematics information (proper motions from PPMXL catalogue - Roeser et al. (2010) and astrometry), we derived Galactic kinematics components : position, velocities and orbits. This allowed me to study the stellar populations in the CoRoT /Exoplanet fields and quantify the metallicity gradient in the Galaxy (Gazzano et al. in preparation). I also participated to the spectroscopic follow-up observations of CoRoT planetary candidates with high resolution spectroscopy, to the spectroscopic analysis, with the VWA software (Bruntt et al. 2010b,a), of planet hosting stars for the CoRoT space mission and to their characterisation and publication. Indeed, the determination of the fundamental parameters of the star is a mandatory step for the complete characterisation of the planet.

L’astérosismologie consiste à sonder les intérieurs stellaires en détectant les ondes sismiques qui s’y propagent. Les géantes rouges, des étoiles évoluées peu massives dont l’hydrogène du cœur est épuisé, sont des pulsateurs de type solaire présentant des modes mixtes qui nous permettent d’accéder directement aux propriétés physiques de leur cœur. Les mesures sismiques disponibles indiquent qu’un ou plusieurs mécanismes physiques encore mal compris contrebalancent l’accélération de la rotation du cœur sous l’effet de sa contraction, en transportant du moment cinétique. La majeure partie de cette thèse a été consacrée au développement d’une méthode permettant une mesure aussi automatisée que possible de la rotation moyenne du cœur des étoiles de la branche des géantes rouges observées par le satellite Kepler (NASA). Les mesures obtenues pour environ 900 étoiles mettent en évidence que la rotation du cœur est à peu près constante le long de la branche des géantes rouges,avec des valeurs indépendantes de la masse des étoiles. Le deuxième volet de cette thèse est consacré à l’interprétation de ces résultats basée sur la modélisation stellaire. L’enjeu consiste à utiliser les mesures à grande échelle obtenues durant la première partie pour caractériser la quantité de moment cinétique qui doit être extraite localement de chaque région du cœur, à différents instants sur la branche des géantes rouges, pour différentes masses stellaires. / Asteroseismology consists in probing stellar interiors through the detection of seismic waves. Red giants are evolved low-mass stars that have exhausted hydrogen in their core. These stars are solar-type pulsators presenting mixed modes that allow us to have a direct access to the physical properties of their core. The available seismic measurements indicate that one or several mechanisms that remain poorly understood counterbalance the acceleration ofthe core rotation, resulting from its contraction, by transporting angularmomentum. The greatest part of this PhD thesis was devoted to the development of a method allowing a measurement as automated as possible of the mean core rotation of stars on the red giant branch that were observed by the Kepler satellite (NASA). The measurements that were derived for almost 900 stars highlight that the core rotation is almost constant along the red giant branch, with values largely independent of the stellar mass. The second part of this PhD thesis is devoted to the interpretation of these results based on stellar modelling. The challenge consists in using the large-scale measurements obtainedin the first part to characterise the quantity of angular momentum that has to be extracted from each layer of the core, at different timesteps on the red giant branch, for different stellar masses.

The significant increase of astronomical data necessitates new strategies and developments to analyse a large amount of information, which no longer is efficient if done by hand. Supervised machine learning is an example of one such modern strategy. In this work, we apply the classification technique on Gaia+2MASS+WISE data to explore the usage of supervised machine learning on large astronomical archives. The idea is to create an algorithm that recognises entries with unusual infrared properties which could be interesting for follow-up observations. The programming is executed in MATLAB and the training of the algorithms in the classification learner application of MATLAB. Each catalogue; Gaia+2MASS+WISE contains ~109, 5×108 and 7×108 (The European Space Agency 2019, Skrutskie et al. 2006, R. M. Cutri IPAC/Caltech) entries respectively. The algorithms searches through a sample from these archives consisting of 765266 entries, corresponding to objects within a <500 pc range. The project resulted in a list of 57 entries with unusual infrared properties, out of which 8 targets showed none of the four common features that provide a natural physical explanation to the unconventional energy distribution. After more comprehensive studies of the aforementioned targets, we deem it necessary for further studies and observations on 2 out of the 8 targets (Nr.1 and Nr.8 in table 3) to establish their true nature. The results demonstrate the applicability of machine learning in astronomy as well as suggesting a sample of intriguing targets for further studies. / Inom astronomi samlas stora mängder data in kontinuerligt och dess tillväxt ökar snabbt för varje år. Detta medför att manuella analyser av datan blir mindre och mindre lönsama och kräver istället nya strategier och metoder där stora datamängder snabbare kan analyseras. Ett exempel på en sådan strategi är vägledd maskininlärning. I detta arbete utnyttjar vi en vägled maskininlärnings teknik kallad klassificering. Vi använder klassificerings tekniken på data från de tre stora astronomiska katalogerna Gaia+2MASS+WISE för att undersöka användningen av denna teknik på just stora astronomiska arkiv. Idén är att skapa en algorithm som identifierar objekt med okontroversiella infraröda egenskaper som kan vara intressanta för vidare observationer och analyser. Dessa ovanliga objekt är förväntade att ha en lägre emission i det optiska våglängdsområdet och en högre emission i det infraröda än vad vanligtvis är observerad för en stjärna. Programmeringen sker i MATLAB och träningsprocessen av algoritmerna i MATLABs applikation classification learner. Algoritmerna söker igenom en samling data bestående av 765266 objekt, från katalogerna Gaia+2MASS+WISE. Dessa kataloger innehåller totalt ~109, 5×108 och 7×108 (The European Space Agency 2019, Skrutskie et al. 2006, R. M. Cutri IPAC/Caltech) objekt vardera. Det begränsade dataset som algoritmerna söker igenom motsvarar objekt inom en radie av <500 pc. Många av de objekt som algoritmerna identifierade som ”ovanliga” tycks i själva verket vara nebulösa objekt. Den naturliga förklaringen för dess infraröda överskott är det omslutande stoft som ger upphov till värmestrålning i det infraröda. För att eliminera denna typ av objekt och fokusera sökningen på mer okonventionella objekt gjordes modifieringar av programmen. En av de huvudsakliga ändringarna var att introducera en tredje klass bestående av stjärnor inneslutna av stoft som vi kallar "YSO"-klassen. Ytterligare en ändring som medförde förbättrade resultat var att introducera koordninaterna i träningen samt vid den slutgiltiga klassificeringen och på så vis, identifiering av intressanta kandidater. Dessa justeringar resulterade i en minskad andelen nebulösa objekt i klassen av ”ovanliga” objekt som algoritmerna identifierade. Projektet resulterade i en lista av 57 objekt med ovanliga infraröda egenskaper. 8 av dessa objekt påvisade ingen av det fyra vanligt förekommande egenskaperna som kan ge en naturlig förklaring på dess överflöd av infraröd strålning. Dessa egenskaper är; nebulös omgivning eller påvisad stoft, variabilitet, Hα emission eller maser strålning. Efter vidare undersökning av de 8 tidigare nämnda objekt anser vi att 2 av dessa behöver vidare observationer och analys för att kunna fastslå dess sanna natur (Nr.1 och Nr.8 i tabell 3). Den infraröda strålningen är alltså inte enkelt förklarad för dessa 2 objekt. Resultaten av intressanta objekt samt övriga resultat från maskininlärningen, visar på att klassificeringstekniken inom maskininlärning är användbart på stora astronomiska datamängder.