02 Décembre – Thesis defense - Yoann Tarricq

10 h Room Univers - Building B18 N / University of Bordeaux (Pessac)

Revisiting the memberships, structural parameters and kinematics of stellar clusters in the Milky Way.

Stellar clusters are gravitationally bound groups of stars. Astronomers distinguish two types of stellar clusters: Gobular and Open Clusters (OCs). This thesis is focused on the latter. The stars belonging to a given OC have all been formed from the same cloud of molecular gas during the same event of star formation. Stars of very different mass are formed during such events but they all share the same kinematics, chemical composition and age. The members of an OC can be spead on vast regions because OCs disrupt during their lifetimes due to their gravitational interactions with their surroundings. The determination of the OCs properties is still much more accurate than for isolated stars as it is based on several stars. This makes OCs efficient probes for Galactic archeology studies, dedicated to unravel the properties and the evolution of our Galaxy.
The unprecedented volume and accuracy of the Gaia mission data dramatically changed our vision of the OC population by giving us access to the three dimensional positions and two dimensional kinematics of more than 1 billion stars. In this thesis, we aim at identifying disrupting clusters and their members and at revisiting the properties of OCs in the light of Gaia data.
We used novel machine learning techniques in order to detect the stars sharing the same kinematics and parallaxes in the surroundings of clusters. Our clustering methodology applied on several hundreds of clusters allowed us to reveal that clusters are not limited to their well studied cores but are more extended than previously expected, regardless of their age. We provide new lists of members which allowed us to identify 70 clusters with remarkable tidal structures, a sign of ongoing disruption. We multiply by more than four the number of such identified structures and highlight the different cluster evaporation processes and the short timescales they need to affect the clusters. We determine several morphological parameters like cluster sizes, radial density profiles and mass segregation levels in a homogeneous and automatic way.
We also combined Radial Velocity (RV) measurements from the Gaia mission and from several ground based surveys to measure the RV of OCs and to study their kinematics in three dimensions. We provide the largest RV catalogue available for OCs, compiled and homogeneised from all available catalogues of high resolution spectroscopy complementary to Gaia. This allows us to characterize the kinematical properties of the OC population and to compare them with that of field stars. Our comparison of the different Galactic components of the velocity shows that Giant Molecular Clouds are less efficient to scatter OCs than field stars. Finally, an analysis of the orbits of the clusters shows that OCs are born in circular orbits, and as age increases they are more prone to suffer perturbations of their orbits.

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