2011ApJ...732...86M

SONYC–Substellar Objects in Nearby Young Clusters–is a survey program to investigate the frequency and properties of substellar objects with masses down to a few times that of Jupiter in nearby star-forming regions. In this third paper, we present our recent results in the Chamaeleon-I star-forming region. We have carried out deep optical and near-infrared imaging in four bands (I, z, J, K_S) using VIMOS on the European Southern Observatory Very Large Telescope (VLT) and SOFI on the New Technology Telescope, and combined our data with mid-infrared data from the Spitzer Space Telescope. The survey covers ∼0.25 deg² on the sky and reaches completeness limits of 23.0 in the I band, 18.3 in the J band, and 16.7 in the K_Sband. Follow-up spectroscopy of the candidates selected from the optical photometry (I ≲ 21) was carried out using the multi-object spectrograph VIMOS on the VLT. We identify 13 objects consistent with M spectral types, 11 of which are previously known M dwarfs with confirmed membership in the cluster. The two newly reported objects have effective temperatures above the substellar limit. We also present two new candidate members of Chamaeleon-I, selected from our JK photometry combined with the Spitzer data. Based on the results of our survey, we estimate that the number of missing very low mass members down to ∼0.008 M_☉ and A_V ≤ 5 is ≤ 7, i.e., ≤ 3% of the total cluster population according to the current census.