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Abstract(s)
As anãs castanhas (BDs) são objectos que fazem a ponte entre as estrelas e os planetas, e cujo processo de formacão é uma das peças cruciais que falta na nossa compreensão de como funciona a formacão de estrelas e planetas. Com esta tese, buscamos responder a algumas das questões mais importantes e ainda não respondidas de forma convincente sobre a formação de BDs, como: Quão comuns são as BDs em relação às estrelas? A eficiência da formação de BDs depende do ambiente de formação estelar? As BDs têm as mesmas propriedades de acreção e evolução que as estrelas de baixa massa?
Na primeira parte da tese, desenvolvemos uma metodologia para derivar o tipo espectral de estrelas de baixa massa e BDs a partir de espectroscopia no infravermelho próximo. Usando técnicas de aprendizagem automática, explorei as características espectrais no infravermelho mais importantes para a classificação da idade e defini um novo índice espectral que pode separar objectos jovens de objectos do campo de baixa massa com um desempenho sem precedentes. Este método é posteriormente aplicado a espectros de KMOS ´ /VLT de candidatos a membros do NGC 2244, um aglomerado massivo e jovem ( 2 Myr) com uma população rica de estrelas massivas. A sua função de massa inicial subestelar e a sua distribuição espacial sugerem que a formação de BD pode ser aumentada na proximidade de estrelas massivas, o que, a confirmar-se, representaria a primeira evidencia forte de um efeito ambiental na formação de BD.
Na parte final da tese estudamos a relação entre os observáveis estelares e do disco protoplanetário em quatro regiões de formação estelar, usando espectros do X-Shooter ˜ /VLT. Descobrimos que as estrelas de baixa massa evoluem mais rapidamente para taxas de acreção mais baixas do que as suas congéneres de massa mais elevada, e que as BDs apresentam escalas de tempo de depleção do disco protoplanetário por acreção mais longas, indicando que a física dos discos em torno de BDs pode ser diferente da física dos discos em torno de estrelas de baixa massa.
Em termos gerais, esta tese contribui para a nossa compreensão da formação e evolução das BDs e oferece conhecimentos valiosos sobre a ligação entre a formação de estrelas e planetas.
Brown dwarfs (BDs) are objects that bridge the realms of stars and planets, and whose formation process is one of the crucial missing pieces in our understanding of how star and planet formation work. With this thesis, we aim at answering some of the most important, and not yet convincingly answered questions of BD formation, namely: How common are BDs with respect to stars? Does the efficiency of BD formation depend on the star-forming environment? Do BDs have the same accretion properties and evolution as low-mass stars? In the first part of the thesis, we developed a methodology to derive the spectral type of low-mass stars and BDs from near-infrared spectroscopy. Using machine learning techniques, I explored the most important infrared spectral features for age classification and defined a new spectral index that separates young from field low-mass objects with unprecedented performance. This is later applied to KMOS/VLT spectra of member candidates of NGC 2244, a massive, young ( 2 Myr) cluster with a rich population of OB stars. Its substellar initial mass function and spatial distribution suggest that BD formation may be enhanced in the proximity of massive stars, which, if confirmed, would represent the first strong evidence of an environmental effect on BD formation. In the final part of the thesis we studied the relationship between the stellar and protoplanetary disk observables in four star-forming regions, using X-Shooter/VLT spectra. We found that low-mass stars evolve faster into lower accretion rates than their higher-mass counterparts, and that BDs present longer accretion depletion timescales indicating that the physics of disks around BDs may be different than around low-mass stars. Overall, this thesis contributes to our understanding of the formation and evolution of BDs and offers valuable insights into the connection between star and planet formation.
Brown dwarfs (BDs) are objects that bridge the realms of stars and planets, and whose formation process is one of the crucial missing pieces in our understanding of how star and planet formation work. With this thesis, we aim at answering some of the most important, and not yet convincingly answered questions of BD formation, namely: How common are BDs with respect to stars? Does the efficiency of BD formation depend on the star-forming environment? Do BDs have the same accretion properties and evolution as low-mass stars? In the first part of the thesis, we developed a methodology to derive the spectral type of low-mass stars and BDs from near-infrared spectroscopy. Using machine learning techniques, I explored the most important infrared spectral features for age classification and defined a new spectral index that separates young from field low-mass objects with unprecedented performance. This is later applied to KMOS/VLT spectra of member candidates of NGC 2244, a massive, young ( 2 Myr) cluster with a rich population of OB stars. Its substellar initial mass function and spatial distribution suggest that BD formation may be enhanced in the proximity of massive stars, which, if confirmed, would represent the first strong evidence of an environmental effect on BD formation. In the final part of the thesis we studied the relationship between the stellar and protoplanetary disk observables in four star-forming regions, using X-Shooter/VLT spectra. We found that low-mass stars evolve faster into lower accretion rates than their higher-mass counterparts, and that BDs present longer accretion depletion timescales indicating that the physics of disks around BDs may be different than around low-mass stars. Overall, this thesis contributes to our understanding of the formation and evolution of BDs and offers valuable insights into the connection between star and planet formation.
Description
Keywords
Anãs castanhas formação de estrelas spectroscopia infravermelha função de massa inicial taxa de acreção de massa Brown dwarfs star formation infrared spectroscopy initial mass function mass accretion rate