SUBSTELLAR OBJECTS IN NEARBY YOUNG CLUSTERS (SONYC) IX: THE PLANETARY-MASS DOMAIN OF CHAMAELEON-I AND UPDATED MASS FUNCTION IN LUPUS-3

Substellar Objects in Nearby Young Clusters-SONYC-is a survey program to investigate the frequency and properties of substellar objects in nearby star-forming regions. We present new spectroscopic follow-up of candidate members in Chamaeleon-I (similar to 2 Myr, 160 pc) and Lupus 3 (similar to 1 Myr, 200 pc), identified in our earlier works. We obtained 34 new spectra (1.5-2.4 mu m, R similar to 600). and identified two probable members in each of the two regions. These include a new probable brown dwarf in Lupus 3 (NIR spectral type M7.5 and T-eff = 2800 K), and an L3 (T-eff = 2200 K) brown dwarf in Cha-I, with masses. below the deuterium-burning limit. Spectroscopic follow-up of our photometric and proper motion candidates in Lupus 3 is almost complete (>90%), and we conclude that there are very few new substellar objects left to be found in this region, down to 0.01-0.02 M. and AV <= 5. The low-mass portion of the mass function in the two clusters can be expressed in the power-law form dN/dM proportional to M-alpha, with alpha similar to 0.7, in agreement with surveys in other regions. In Lupus 3 we observe a possible flattening of the power-law IMF in the substellar regime: this region seems to produce fewer brown dwarfs relative to other clusters. The IMF in Cha-I shows a monotonic behavior across the deuterium-burning limit, consistent with the same power law extending down to 4-9 Jupiter masses. We estimate that objects below the deuterium-burning limit contribute of the order of 5%-15% to the total number of Cha-I members.