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 (~2 Myr, 160 pc) and Lupus 3 (~1 Myr, 200 pc), identified in our earlier works. We obtained 34 new spectra (1.5–2.4 μm, R ~ 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_Θ and A_V ≤ 5. The low-mass portion of the mass function in the two clusters can be expressed in the power-law form dN/dM α M^-α, with α∼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.