A sharpened close-up of R136 and NGC3603: unshrouding the nature of their stellar population

The aim of this thesis is to understand the different aspects of the dynamical and stellar evolution of very massive clusters. These newly formed massive clusters are the most important link between the formation of the massive star clusters and their evolution. We chose two young massive clusters, located in the most massive Galactic/extra-Galactic HII regions, NGC3603 and R136 which hosts the most massive stars in the local universe. This thesis contains the photometric analysis of the core of R136 and NGC3603 using the Hubble Space Telescope (HST) data in the visible and the Very Large Telescope (VLT/SPHERE) in the infrared. Thanks to SPHERE extreme AO system, I detected many faint low-mass stars for the first time in the core of both clusters in the vicinity of massive bright objects. Comparing the results of the HST and SPHERE photometry analysis, NGC3603 shows no signature of mass segregation in its core for two main reasons: First, the MF slope in the very core is not flatter than the next radial bin. Second, both slopes are similar to the MF values found in previous works for the outer regions. R136 is partially resolved in SPHERE/IRDIS data. The majority (above 90%) of massive stars (brighter than 17 mag in K and 16 mag in J) have visual companions closer than 0.2”. Among them, R136a1 and R136c have visual companions detected for the first time. R136a3 is resolved as two stars (PSF fitting). Considering the spectroscopic and photometric errors on the extinction (A = 0.45 ± 0.5 and A = 0.2 ± 0.5) and the age (1.8+1.2 Myr) of the J K 0.8 cluster members, we estimate a mass range for each detected star. The generalized histogram of stellar masses (MF) was plotted at different ages with given errors on each stellar mass. We created series of simulated images of R136 from the output of Nbody6 code, to be compared with the HST/WFPC2 data of R136. These numerical simulations are done in order to check the effect of initial binarities, mass segregation and stellar evolution on the dynamical evolution of R136-like clusters at the early stages of their life (4 Myr). The analysis of the bright stellar population (spectroscopic and photometry), shows that we need more resolution to go further on studying R136 which is 7-8 times further than NGC3603. The spectroscopic analysis find very massive stars in the core of R136 where, for some of them SPHERE data has resolved them visually. Future instruments with higher angular resolution (milliarcsec) like E-ELT (MI- CADO), JWST and VLTI/GRAVITY can resolve the compact core of R136 and similar objects at further distances. The kinematics of the core of these compact clusters can be studied by using astrometric analysis with μarcsec accuracy with GRAVITY in near future.See published papers for update