Discovery and characterization of substellar companions in stellar clusters using Karhunen-Loève Image Processing of HST data

We present the main results of this thesis project, consisting in the development and application of a pipeline able to extract accurate stellar photometry and recover faint companions embedded point spread function of their host stars in HST/ACS and WFC3-IR images through Karhunen-Loeve Image Processing (KLIP) analysis. The pipeline has been used to disentangle the population of binaries and isolated stars in the Orion Nebula Cluster in a range of separations 0.12–1.95 arcsec, primary masses 0.015–1.6 Msun and companion masses 0.004–1 Msun. We have analyzed the data from two HST Treasury programs (GO-10246 and GO-13826, P.I. M. Robberto) spanning a wide range of filters from visible bands to infrared (i.e., filters F435W, F555W, F658N, F775W, F850LP, F130N, and F139M). A Bayesian approach with MCMC strategy has been adopted to estimate three fundamental parameters through SED fitting: mass, age, and extinction. An analysis of the age distribution for isolated stars in the cluster unveiled the presence of two different populations with an average age of ~0.85 and 1.6 Myr, supporting the idea of at least two discrete episodes of star formation over the first few Myr. Using F658N filter photometry we have estimated the accretion luminosity (Lacc) and the mass accretion rate (dMacc) for a sample of ~700 bonafide isolated cluster sources. The dMacc–Mstar relationship exhibits two different regimes: the more massive stars (Mstar > 0.3 Msun) display a rather shallow linear dependence on the stellar mass in agreement with models of centrally irradiated accretion disks around solar-mass stars. Lower mass stars instead (Mstar ≤ 0.3 Msun) can be divided in two different populations: 1) one that nicely matches the population of high mass objects, extending the trend to lower mass stars; 2) a second population that, departing at Mstar ~ 0.3 Msun, displays a steeper relation between dMacc and Mstar. In what concerns binarity, a new catalog of 110 binaries has been compiled, of which 23 from our KLIP analysis, providing a multiplicity fraction for the cluster of 13% ± 1%. Compared to other star-forming regions, the multiplicity fraction for this cluster is ~2 times smaller than e.g., Taurus, while it is still compatible with the value observed in the field over a similar range of masses and separations. An analysis of the distribution of wide (separation > 0.6 arcsec) over close (separation ≤ 0.6 arcsec) binaries as a function of the distance from the center of the cluster, reveals clear signs of early dynamical evolution in the cluster.