The Initial Mass Function of the Orion Nebula Cluster from Near-Infrared Photometry

The main goal of this thesis is the complete characterization of the IMF of the Orion Nebula Cluster (ONC) down to the Brown Dwarfs (BDs) regime, using ground-based Near-Infrared (NIR) photometric observations. The data taken in the framework of the Hubble Space Telescope (HST) Treasury Program on the ONC have been obtained with the wide-field imager Infrared Side Port Imager (ISPI) at the Blanco 4m telescope of CTIO, and cover an area of about 0.25 square degrees roughly centered on Theta1OriC. We observed the region in the JHKS bands with exposure times of 330 s. As a result of our survey, we provide 2MASS-calibrated astrometry and photometry for ~7000 sources in the ONC region. Analyzing our photometric sample, we derive the contamination-completeness corrected (J,J-H) CMDs of the ONC, canceling out the contribution from the contaminant population. We also develop a statistical algorithm, which combines the CMD of the ONC with our reference isochrone and, taking into account the presence of extinction and NIR excess, derives the intrinsic Luminosity Function (LF) of the cluster. We finally combine the LF with our empirical NIR isochrone to derive a statistical estimate of the IMF at different distances from the cluster center. We find that the mass distribution of the cluster is peaked at ~0.16 M and falls off crossing the hydrogen burning limit, continuously decreasing in the BDs domain down to ~0.03 M . We also find that the substellar-to-stellar objects ratio in the ONC decreases with increasing distance from the cluster center, suggesting that BDs are preferentially formed in the deep gravitational potential well where the most massive stars of the Trapezium cluster are also found.