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Formation d'étoiles massives en amas : conditions initiales, origine des masses stellaires et éjections protostellaires

Abstract : The research I have performed during my PhD addresses three major challenges of the star formation field: constraining, observationally, the earliest phases of high-mass star formation – the so-called prestellar stage, studying the origin of the stellar masses, and characterizing the process of protostellar accretion-ejection.Going beyond the current paradigms of star formation requires studying star-forming regions which are more representative of the general mode of star formation in the Milky Way. To this purpose I have used ALMA observations of W43-MM1, a young located at 5.5 kpc distance from the Sun, which presents a high star formation rate. The first step of my work has been to identify and characterize cores in the continuum image. I discovered 131 cores about 2400 AU in size which have mass between 1 and 100 Msun. I measured their mass distribution (CMF) and found a slope of -0.96 +/- 0.13 on 1.6 - 100 Msun that is markedly flatter than the reference Salpeter slope of the IMF on that range, -1.35. This means an overabundance of high-mass cores - and thus high-mass stars -compared to the number expected by the current models of star formation. Possible explanations imply either that star-formation is atypical in W43-MM1 (variably in the Milky Way) or that high-mass stars form at different time than low-mass stars in a cluster (star formation would not be a continuous process).I have characterized these cores using CO(2-1) and SiO(5-4) lines and revealed a rich cluster of 45 outflow lobes from 27 cores covering the whole mass range and including 11 high-mass cores (M>16 Msun). I have also used the detection of Complex Organic Molecules (COMs), typically detected in warm environments like hot cores, as indicator of the protostellar activity. 12 out of the 13 high-mass cores in W43-MM1 have eventually been characterized as protostellar, leaving one good high-mass prestellar core candidate. These statistics raises question about the universality of a prestellar phase for high-mass stars and suggests that the core-fed models for high-mass star formation cannot generally apply. The protostellar outflows also bring valuable information on the accretion/ejection history. I have studied the kinematics of high-velocity molecular jets that divide into knots using Position-Velocity diagrams. I have shown that the complex velocity structures of these knots hide a strong variability, and evaluated the associated timescale between two ejections to be about 500 yr. This is reminiscent of the values obtained between accretion burst in FU Orionis stars.
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Submitted on : Wednesday, April 1, 2020 - 12:54:15 PM
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Thomas Nony. Formation d'étoiles massives en amas : conditions initiales, origine des masses stellaires et éjections protostellaires. Astrophysique stellaire et solaire [astro-ph.SR]. Université Grenoble Alpes, 2019. Français. ⟨NNT : 2019GREAY054⟩. ⟨tel-02527680⟩



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