abstract: The Kuiper Belt Object Haumea is one of the most fascinating objects in the solar system. Spectral reflectance observations reveal a surface of almost pure water ice, yet it has a mass of 4.006 × 1021 kg, measured from orbits of its moons, along with an inferred mean radius of 715 km, and these imply a mean density of around 2600 kg m−3. Thus the surface ice must be a veneer over a rocky core. This model is supported by observations of Haumea's light curve, which shows large photometric variations over an anomalously rapid 3.9154-hour rotational period. Haumea's surface composition is uniform, therefore the light curve must be due to a varying area presented to the observer, implying that Haumea has an oblong, ellipsoidal shape. If Haumea's rotation axis is normal to our line of sight, and Haumea reflects with a lunar-like scattering function, then its axis ratios are p = b/a = 0.80 (in the equatorial cross section) and q = c/a = 0.52 (in the polar cross section). In this work, I assume that Haumea is in hydrostatic equilibrium, and I model it as a two-phase ellipsoid with an ice mantle and a rocky core. I model the core assuming it has a given density in the range between 2700–3300 kg m−3 with axis ratios that are free to vary. The metric which my code uses calculates the angle between the gravity vector and the surface normal, then averages this over both the outer surface and the core-mantle boundary. When this fit angle is minimized, it allows an interpretation of the size and shape of the core, as well as the thickness of the ice mantle. Results of my calculations show that Haumea's most likely core density is 2700–2800 kg m−3, with ice thicknesses anywhere from 12–32 km over the poles and as thin as 4–18 km over the equator. / Dissertation/Thesis / Masters Thesis Astrophysics 2015

Les occultations stellaires sont une méthode puissante pour étudier les petits corps du Système Solaire, trop faibles ou lointains pour être étudiés par d’autres techniques. Cette méthode consiste à observer le corps passant devant une étoile d’arrière plan. Le flux lumineux stellaire est alors intercepté quelques instants, ce qui permet de caractériser la forme de l’objet et d’étudier son environnement (anneau, atmosphère ou satellite). Deux anneaux denses et étroits ont été découverts en Juin 2013 pour la première fois autour du plus gros des Centaures Chariklo (corps central : 250 km de diamètre ; anneaux : 800 km). Cette thèse présente les 16 occultations par Chariklo et ses anneaux observées depuis 2013. Ces données représentent les seules observations aujourd’hui existantes de ce système planétaire. Une première étude de leur structure et de leur géométrie est détaillée dans ces travaux. Mes travaux ont également porté sur une occultation par la planète naine Haumea observée en Janvier 2017. Celle-ci nous a révélé la présence d’un anneau dense de rayon ~2281 km autour de ce corps tri-axial. Haumea est donc le deuxième corps en dehors des planètes géantes à posséder un anneau. Cette occultation permet notamment de déduire la forme, la taille (1161x852x513 km) et la densité d’Haumea. Cette seconde découverte tend à montrer que les anneaux autour des petits corps ne sont pas si rares. La publication du catalogue Gaia DR1 en 2016 a permis d’améliorer considérablement la précision de nos prédictions donc nos observations. Les prochaines années seront consacrées à scanner plus en détails les anneaux de Chariklo et d’Haumea, mais aussi à chercher des anneaux autour d’autres petits corps afin de comprendre les mécanismes de formation et d’évolution de ces systèmes. / Stellar occultations are a very powerful tool to study small bodies of the Outer Solar System, which are usually too faint or too far away to be studied by others techniques. They occur when a body passes in front of a background star. The stellar flux will then be intercepted during several seconds/minutes. Observation of the occulted star in time allows us to study the shape and size of the occulting object and its vicinity (ring, atmosphere or satellite). For the first time, two dense and narrow rings have been discovered on June 2013 around the biggest known Centaur Chariklo (body’s diameter: 250 km and rings diameter: 800 km). My work shows the 16 observed occultations by Chariklo and/or its rings since 2013. Those data are the only one available about this planetary system today. A first study of their structure and their geometry is conducted. Another part of my work was to study an occultation by the dwarf planet Haumea recorded on January 2017. It revealed the presence of a ring of radius ~2281 km around this tri-axial body. This discovery made Haumea the second body with a ring orbiting around except the four giant planets. This occultation allows us to derive the shape, size (1161x852x513 km) and density of the main body. This second discovery tends to show that rings in small planetary systems are more common that we thought. The release of Gaia DR1 catalog in 2016 greatly improved our predictions of such occultations. In the forthcoming years, efforts will be made on detailed and localized studies of Chariklo’s and Haumea’s rings, but also on search for other small rings systems in order to better understand the origins and the evolution mechanisms of such systems.

Submitted by Gabriel Borderes Motta (gabriel_borderes@yahoo.com.br) on 2018-05-08T22:40:01Z No. of bitstreams: 1 Tese.pdf: 94594231 bytes, checksum: 424f474ffb856276cc3c8fc7f0e81790 (MD5) / Approved for entry into archive by Pamella Benevides Gonçalves null (pamella@feg.unesp.br) on 2018-05-10T13:22:38Z (GMT) No. of bitstreams: 1 borderes-mota_g_dr_guara.pdf: 94594231 bytes, checksum: 424f474ffb856276cc3c8fc7f0e81790 (MD5) / Made available in DSpace on 2018-05-10T13:22:38Z (GMT). No. of bitstreams: 1 borderes-mota_g_dr_guara.pdf: 94594231 bytes, checksum: 424f474ffb856276cc3c8fc7f0e81790 (MD5) Previous issue date: 2018-03-06 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Em geral, pequenos corpos do sistema solar, como asteroides e cometas, têm uma forma muito irregular, o que afeta significativamente o seu potencial gravitacional, dificultando os estudos da dinâmica ao redor destes corpos. Uma primeira aproximação é a expansão em harmônicos esféricos, onde os termos C20 e o C22 caracterizam a irregularidade do corpo. Usamos essa aproximação em superfícies de secção de Poincaré para estudar as regiões próximas ao planeta anão Haumea, onde foi observado um anel. A partir do mapeamento feito pela técnica de superfície de secção de Poincaré, foi possível identifi- car Famílias de órbitas periódicas e regiões estáveis. Duas Famílias de órbitas periódicas foram destacadas, a primeira uma Família de segundo tipo associada à ressonância 1:3 (Família ressonante) e a segunda uma Família de primeiro tipo (Família central). As simulações indicam que as partículas do anel podem estar em órbitas da Família central. Já a Família ressonante, não pode ser responsável pelo anel devido a excentricidade de suas órbitas e da sua posição. Para simular de forma mais realista a irregularidade de um pequeno corpo, é usada uma melhor aproximação para o cálculo do potencial gravitacional. O modelo de concentração de massa, ou modelo de mascons, é uma aproximação discreta da forma de um corpo, capaz de simular um potencial irregular, assimétrico e tridimensional. A esse modelo é aplicada a superfície de secção de Poincaré, com o objetivo de estudar a dinâmica da região próxima ao asteroide 4179 Toutatis. Quatro Famílias de órbitas periódicas são destacadas e estudadas. Uma Família é de primeiro tipo e as outras três são de segundo tipo associadas às ressonâncias 3:1, 2:1 e 2:3. Apesar do potencial gravitacional tridimensional ser adotado em uma ferramenta usualmente bidimensional, é possível analisar como um problema bidimensional quando a variação na terceira dimensão é baixa. Estudando em conjunto as superfícies de secção de Poincaré e a variação máxima na terceira dimensão, verifica-se a estabilidade ou não das trajetórias simuladas / In general, small bodies of the Solar system, e.g. asteroids and comets, have a very irregular shape. This feature affects significantly the gravitational potential around these irregular bodies, which hinders dynamical studies. A first approximation is an expansion in spherical harmonics, where C20 and C22 characterize the irregularity of the body. This approach is used on Poincaré surfaces of sections to study regions close to the dwarf planet Haumea. This regions are where the observed ring. By the technique of Poincaré surface of section, it was identified Families of periodic orbits and stable regions. Two Families of periodic orbits were studied, the first Family is a second type associated with the 1:3 resonance (resonant Family) and the second Family is a first type (central Family). During the simulations the ring particles can be in orbits of the central Family. But the resonant Family can not be responsible for the ring due the eccentricity and position of their orbits. In order to more realistically simulation of the irregularity of the body, a better approximation is necessary for the computation of the gravitational potential. The mass concentration model, or mascon model, is a discrete approximation of the shape of a body. This model simulates an irregular, asymmetric and three-dimensional potential. This model was applied in a Poincaré surfaces of section, mainly to study the dynamics of the region close to the asteroid 4179 Toutatis. Four Families of periodic orbits were studied. One of then is a first type and the others were the second type and associated with the resonances 3:1, 2:1 and 2:3. Although the three-dimensional gravitational potential is adopted in a usually two-dimensional tool, it is possible to analyze as a two-dimensional problem when the variation in the third dimension is low. By a analyzing of the Poincaré surfaces of section and a maximum variation in the free dimension together, the stability of the simulated trajectories is measured