Titan, le plus grand satellite de Saturne, possède une atmosphère dense, majoritairement composée d’azote (N_2) et de méthane (〖CH〗_4) et qui s’étend sur environ 1500 km d’altitude. L’interaction entre ces espèces et le rayonnement solaire ouvre la voie à des réactions de photodissociation et de photoionisation qui constituent le point de départ d’une croissance moléculaire très rapide et efficace dès les plus hautes couches de l’atmosphère. Cette croissance aboutie à la synthèse d’aérosols vers 1000 km d’altitude qui précipitent ensuite dans l’atmosphère.L’objectif de cette thèse est, de s’intéresser expérimentalement à l’interaction entre les espèces peuplant la haute atmosphère de Titan et le rayonnement solaire énergétique dans la gamme de l’UV sous vide (VUV), (longueur d’onde < 150 nm). Tout d’abord, nous avons étudié l’interaction avec les petites espèces chimiques neutres et majoritaires grâce à une nouvelle source VUV spécialement conçue pour cette thèse couplée au réacteur photochimique APSIS. Ce dispositif met en lumière la photochimie du système couplé azote-méthane, encore très mal connue.Ensuite, nous nous sommes penchés sur l’effet du VUV sur les aérosols formés dès les plus hautes couches, en photoionisant avec le rayonnement synchrotron de la ligne DESIRS des analogues de ces aérosols produits avec le dispositif PAMPRE. Cette méthode permet d’obtenir des informations sur leur photoionisation (seuil et section efficaces, spectres de photoélectrons) ainsi que sur leurs propriétés optiques. / Titan, Saturn's biggest satellite, possesses a dense atmosphere, mainly composed of nitrogen (N2) and methane (CH4), which goes up to 1500 km in altitude. The interaction between those chemical species and the solar light leads to some photodissociation and photoionization reactions that are the starting point of a fast and efficient molecular growth in the upper layers of the atmosphere. This growth ends with the synthesis of aerosols around an altitude of 1000 km that will then precipitate into the atmosphere.The aim of this thesis is to experimentally study the interaction between the species present in the upper atmosphere and the energetic solar radiations in the vacuum ultraviolet (VUV) range (wavelength below 150 nm). First, we looked at the interaction with the small, neutral and most abundant species thanks to a new VUV source specially designed for this thesis and coupled to photochemical reactor called APSIS. This new platform sheds some light on the photochemistry of a coupled nitrogen-methane system, which remains poorly understood.Then, we focused on the effects of the VUV light on the aerosols formed in the upper layers, by photoionizing, with synchrotron light from the DESIRS beamline, analogs of those aerosols produced on the PAMPRE platform. This method gives information about their photoionization (threshold and cross sections, photoelectron spectra) but also about their optical properties.

Cool giant and supergiant stars are among the largest and most luminous stars in the Universe and, therefore, dominate the integrated light of their host galaxies. These stars were extensively studied during last few decades, however their relevant properties like photometric variability and mass loss are still poorly constrained. Understanding of these properties is crucial in the context of a broad range of astrophysical questions including chemical enrichment of the Universe, supernova progenitors, and the extragalactic distance scale. Atmospheres of giant and supergiant stars are characterized by complex dynamics due to different interacting processes, such as convection, pulsation, formation of molecules and dust, and the development of mass loss. Current 1D/3D dynamical model atmospeheres are able to simulate these processes and produce a good agreement with the observed spectral features of evolved stars. However, the models lack constraints and need to be confronted to observables. Dynamical processes in stellar atmospheres impact the formation of spectral lines producing their asymmetries and Doppler shifts. Thus, by studying the line-profile variations on spatial and temporal scales it is possible to reconstruct atmospheric motions in evolved stars. As will be shown in this thesis, a tomographic method is an ideal technique for this purpose. The tomographic method is based on construction and cross-correlation of spectral templates (masks) with observed or synthetic stellar spectra in order to recover velocity fields at different optical depths in the stellar atmosphere.The first part of the thesis further improves the original implementation of the tomographic method. This improvement involves the computation of the contribution function in order to correctly determine an optical depth of formation of spectral lines. The tomographic method is, then, fully validated by applying it to a stellar convection simulation of a red supergiant star and correctly recovering its velocity field throughout the atmosphere. The second part of the thesis applies the tomographic method to the red supergiant star μ Cep in order to constrain its atmospheric motions and relate them to photometric variability. A phase lag (hysteresis) between the effective temperature and the radial velocity variations is revealed with timescales of a few hundred days, similar to photometric ones. A comparison to a stellar convection simulation of a red supergiant star indicates that hysteresis loops are linked to the stochastic shocks generated and shaped by the underlying large-scale convection and may be responsible for photometric variations in μ Cep. The third part of the thesis applies the tomographic method to spectro-interferometric observations of the Mira-type star S Ori. The uniform-disk angular diameters measured at wavelengths contributing to the tomographic masks increase with decrease of an optical depth probed by the masks. This validates the capability of the tomographic method to probe distinct geometrical depths in the stellar atmosphere. The last part of the thesis applies the tomograhic method to the Mira-type star RY Cep and compares the results to those obtained for μ Cep in this thesis. The comparison reveals differences in their behavior in the temperature-velocity plane pointing to the posibility to differentiate between Mira-type and red supergiant stars from their spectroscopic signatures. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

The transit method, employed by Microvariability and Oscillation of Stars (MOST), Kepler, and various ground-based surveys has enabled the characterization of extrasolar planets to unprecedented precision. These results are precise enough to begin to measure planet atmosphere composition, planetary oblateness, starspots, and other phenomena at the level of a few hundred parts per million. However, these results depend on our understanding of stellar limb darkening, that is, the intensity distribution across the stellar disk that is sequentially blocked as the planet transits. Typically, stellar limb darkening is assumed to be a simple parameterization with two coefficients that are derived from stellar atmosphere models or fit directly. In this work, we revisit this assumption and compute synthetic planetary-transit light curves directly from model stellar atmosphere center-to-limb intensity variations (CLIVs) using the plane-parallel Atlas and spherically symmetric SAtlas codes. We compare these light curves to those constructed using best-fit limb-darkening parameterizations. We find that adopting parametric stellar limb-darkening laws leads to systematic differences from the more geometrically realistic model stellar atmosphere CLIV of about 50–100 ppm at the transit center and up to 300 ppm at ingress/egress. While these errors are small, they are systematic, and they appear to limit the precision necessary to measure secondary effects. Our results may also have a significant impact on transit spectra.

This report will first give a brief background on asymptotic giant branch (AGB) stars and the characteristics that make them interesting to study. Some methods and tools used in the field are then introduced, before the photometric variability of these stars is investigated. This is achieved by using data from dynamical models of AGB stars with differing chemical abundances. The R, J and K bands of the UBVRI system are specifcally investigated to explore whether these are good candidates for AGB photometric and spectroscopic research. Lastly, the molecular features at these wavelengths are investigated to understand the impact that they have on the photometric variability during the pulsation cycle and which molecules are most prominent in this.

Using high resolution optical spectra from Mount John University Observatory, Mount Stromlo Observatory and the Anglo-Australian Observatory, new, high accuracy radial velocity curves have been obtained for the two bright southern Cepheids l carinae (HR 3884) and beta doradus (HR 1922). An indepth investigation into period variations, cycle-to-cycle and long-term variations in the velocity curves and the reliability of the combination of velocity data from different observatories is carried out. Evidence for shock waves in the atmosphere of l car and resonance in beta dor is discussed. A grid of static model atmospheres incorporating plane-parallel geometry is compared with the observational spectra of both Cepheids, using line depth ratios, to determine the variation in effective temperature, surface gravity and microturbulence with phase. This information is used to determine the phase dependence of the surface-brightness for both Cepheids. The surface brightness variation with phase was found to follow an almost linear relationship. The distance to and radius of the Cepheids are determined using both a near-infrared version of the Barnes-Evans method and the Fourier Baade-Wesselink (BW) method. The derived radii and distances agree within the limits of the errors for both methods. The Fourier BW method was found to be very sensitive to phase shifts between the photometric and spectroscopic data and the derived distance highly dependent on the assumed reddening. An investigation into line profile variations in l car and beta dor has revealed the magnitude of these phenomena increase as the pulsational period of the Cepheid increases. It is estimated that line level variations introduce an additional uncertainty into derived radii of approximately 4 per cent for beta dor and 10 per cent in lcar. The uncertainty introduced into derived distances and radii by line profile asymmetries was estimated to be of the order of 6 per cent in beta dor and 10 per cent in l car. A comparative analysis is made of the hydrogen line radial velocity curves of l car and beta dor. A trend in the properties of these radial velocity curves with period has been revealed. In longer period Cepheids, the Halpha line seems to be forming in a region that does not partake in the pulsation as a whole, probably in a chromospheric shell. A quantitative analysis of the asymmetries in these lines reveal large redward asymmetries near maximum infall velocity. The magnitude of these asymmetries and the period for which they are present are larger in l Car than in beta dor. The blueward asymmetries in the Halpha line in l Car are comparable in magnitude to the redward asymmetries while the other lines exhibit only small blueward asymmetries. A qualitative analysis of these line profiles with phase reveal no conclusive evidence for line doubling in these Cepheids. Evidence of emission is found in the Halpha and H Beta lines of beta dor and l car. The strength and duration of the emission is found to be greater in the longer period Cepheid. Although it is likely that this emission is shock-related, theoretical work is needed to determine the exact origin of the emission. A non-LTE radiative hydrodynamic model for l Car has been created. This atmosphere will be used in further work to calculate synthetic spectral line profiles which will aid the interpretation of our observational results.

L'étude comparative des atmosphères planétaires est une science jeune, pleine d'intérêt du fait de la diversité des situations observées Dans ce cadre, l'exploration de la planète Mars est actuellement un enjeu important. Ce travail de thèse concerne plus particulièrement la double modélisation de l'ionosphère martienne et de son rayonnement dans Ie domaine ultraviolet. Le modèle de l'ionosphère de Mars est basé sur un modèle ionosphérique terrestre couplé cinétiquejfluide. Sont notamment décrits les profils de production et de concentration des ions H+, 0+, at et cot et des électrons thermiques, et les températures des électrons. Le modèle reproduit convenablement les mesures du module Viking 1. Pour nous familiariser avec l'étude d'un rayonnement, nous avons développé un modèle d'émission de deux raies de l'oxygène atomique (558 et 630 nm), validé pour la Terre par des comparaisons avec des mesures d'un interféromètre embarqué. Fort de cette expérience, un modèle de l'émission lumineuse .des ions cot (288 nm) et de l'oxygène atomiaue (135 et 297 nm) dans l'atmosphère martienne est présenté. Les comparaisons avec les mesures des spectromètres des sondes Mariner 6 et 7 sont satisfaisantes.

Les etoiles massives jouent un role crucial dans divers domaines de l'astrophysique: elles produisent la majorite des elements plus lourds que l'Oxygene; elles creent des regions HII grace a leur fort pouvoir ionisant; elles terminent leur vie en supernovae. Par ailleurs, elles ont la particularite d'emettre continument un fort vent engendre par la pression de radiation et qui interagit avec le milieu interstellaire environnant, creant des bulles, des chocs et declenchant l'effondrement des nuages moleculaires voisins. En outre, la perte de masse associee a ces vents est telle qu'elle va gouverner completement l'evolution de l'etoile. Il est donc important de connaitre quantitativement ces vents radiatifs d'etoiles massives, ce qui passe par la modelisation de leurs atmospheres. Dans ce cadre, les codes de calcul ont vu recemment leur fiabilite se renforcer significativement grace a l'inclusion du line-blanketing, permettant ainsi d'esperer une connaissance accrue des proprietes des etoiles massives. Dans une premiere partie de cette these, nous nous sommes donc interesses a l'etude des effets induits par le line-blanketing sur le comportement des modeles d'atmosphere. Nous avons ainsi montre qu'a la fois le spectre emergent et la structure de cette atmosphere etaient modifies. Cela se traduit par une reduction de l'echelle de temperature (relation Teff - type spectral) des etoiles O de 1500 a 4000 K pour les naines de type spectral tardif et precoce respectivement, ainsi que par une revision des parametres fondamentaux (luminosite, flux ionisants). L'inclusion des metaux modifie egalement de facon significative la distribution spectrale d'energie de ces etoiles, ce qui permet entre autre de mieux rendre compte des sequences d'excitation construites a partir de raies nebulaires infrarouges observees dans des regions HII compactes. Dans une seconde partie, nous nous sommes interesses aux proprietes de vent des etoiles de type O. Une premiere etude menee sur les etoiles de la region N81 du SMC au moyen de spectres STIS/HST a revele l'exceptionnelle faiblesse de leurs vents ainsi que leur probable appartenance a la classe spectrale Vz. Avec des taux de perte de mass de l'ordre de 1e-9 Msol/an, ces vents sont en effet plus faibles que ce qui a jamais ete observe et predit par les simulations hydrodynamiques. Il en va de meme pour les quantites de mouvement modifiees. Une seconde etude d'etoiles Galactiques connues pour montrer qualitativement des vents faibles ainsi que de quelques etoiles Vz a elle aussi revele l'existence de vents faibles dans les etoiles les moins lumineuses, revelant de ce fait une rupture de pente de la relation quantite de mouvement modifiee - luminosite pour des luminosites plus faibles que 1e5.2 Lsol. L'origine de la faiblesse de ces vents reste inconnue puisque ni la metallicite ni la jeunesse des etoiles ne semblent etre en mesure de les expliquer.

The study of song cycles poses difficulties for both analysts and performers. These challenges stem largely from two qualities intrinsic to the genre: (1) the inclusion of two semiotic systems, language and music, and (2) the use of multi-movement structures. Several scholars have addressed these issues; however, a model built on a balanced consideration of both text-based/dramatic events and purely musical elements, has yet to be offered. This study proposes such a model with separate applications for both performers and analysts. Focusing on the identification of features connecting song cycles in their entirety, deep voice-leading associations and movements in key paths are examined in the application for analysts, whereas the performers’ application concentrates on recognizing underlying “atmospheres” and forms of acceleration. Each application is applied to Schumann’s Frauenliebe und –leben and Dichterliebe song cycles, demonstrating the benefits of employing this model in the development of both performative and analytical interpretations.

The study of song cycles poses difficulties for both analysts and performers. These challenges stem largely from two qualities intrinsic to the genre: (1) the inclusion of two semiotic systems, language and music, and (2) the use of multi-movement structures. Several scholars have addressed these issues; however, a model built on a balanced consideration of both text-based/dramatic events and purely musical elements, has yet to be offered. This study proposes such a model with separate applications for both performers and analysts. Focusing on the identification of features connecting song cycles in their entirety, deep voice-leading associations and movements in key paths are examined in the application for analysts, whereas the performers’ application concentrates on recognizing underlying “atmospheres” and forms of acceleration. Each application is applied to Schumann’s Frauenliebe und –leben and Dichterliebe song cycles, demonstrating the benefits of employing this model in the development of both performative and analytical interpretations.

The sources and sinks of energy for transient waves in the Martian atmosphere are investigated, applying diagnostic techniques developed for the analysis of terrestrial baroclinic waves to output from a Mars General Circulation Model. These diagnostic techniques include the vertically averaged eddy kinetic energy and regression analysis. The results suggest that the primary source of the kinetic energy of the waves is baroclinic energy conversion in localized regions. It is also shown that there exist preferred regions of baroclinic energy conversion. In addition, it is shown that downstream baroclinic development plays an important role in the evolution of the waves and in the baroclinic energy conversion process. This is the first time that evidence for downstream baroclinic development has been found for an atmosphere other than the terrestrial one.