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Theses

COSMIC-DANCE : A comprehensive census of nearby star forming regions

Abstract : Understanding how stars form is one of the fundamental questions which astronomy aims to answer. Currently, it is well accepted that the majority of stars form in groups and that their predominant mechanism of formation is the core-collapse. However, several mechanisms have been suggested to explain the formation of substellar objects, and their contribution is still under debate. The main goal of this thesis is to determine the initial mass function, the mass distribution of stars at birth time, in different associations and star-forming regions. The mass function constitutes a fundamental observational parameter to constrain stellar and substellar formation theories since different formation mechanisms predict different fraction of stellar and substellar objects. We used the Gaia Data Release 2 catalogue together with ground-based observations from the COSMIC-DANCe project to look for high probability members via a probabilistic model of the distribution of the observable quantities in both the cluster and background populations. We applied this method to the 30 Myr open cluster IC 4665 and the 1 - 10 Myr star-forming region Upper Scorpius (USC) and r Ophiuchi (r Oph). We found very rich populations of substellar objects which largely exceed the numbers predicted by core-collapse models. In USC, where our sensitivity is best, we found a large number of free-floating planets and we suggest that ejection from planetary systems must have a similar contribution than core-collapse in their formation. The age is a fundamental parameter to study the formation and evolution of stars and is essential to accurately convert luminosities to masses. For that, we also presented a strategy to study the dynamical traceback age of young local associations through an orbital traceback analysis. We applied this method to determine the age of the b Pictoris moving group and in the future, we plan to apply it to other regions such as USC. The members we identified with the membership analysis are excellent targets for follow-up studies such as a search for discs, exoplanets, characterisation of brown dwarfs and free-floating planets. I this thesis, we presented a search for discs hosted by members of IC 4665 and we found six excellent candidates to be imaged with ALMA or the JWST. The tools we developed, are ready to be used in other regions such as USC and r Oph, where we expect to find a larger number of disc-host stars.
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Núria Miret Roig. COSMIC-DANCE : A comprehensive census of nearby star forming regions. Astrophysics [astro-ph]. Université de Bordeaux, 2020. English. ⟨NNT : 2020BORD0327⟩. ⟨tel-03189124⟩

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