Substellar Objects in Nearby Young Clusters (SONYC) IX: The planetary-mass domain of Chamaeleon-I and updated mass function in Lupus-3

TL;DR

This study presents spectroscopic follow-up of candidate substellar objects in Chamaeleon-I and Lupus-3, revealing new brown dwarfs, analyzing the mass function, and suggesting regional differences in brown dwarf formation.

Contribution

It provides new spectroscopic confirmations of substellar objects, updates the mass function in two regions, and identifies potential regional variations in brown dwarf populations.

Findings

Two new probable brown dwarfs identified in each region.

The mass function follows a power-law with α~0.7, consistent with other regions.

Lupus 3 shows a possible flattening of the IMF in the substellar regime.

Abstract

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 mum, 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 Teff=2800 K), and an L3 (Teff=2200 K) brown dwarf in Cha-I, with the mass 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 MSun and Av \leq 5. The low-mass portion of the mass function in the two clusters can be expressed in the power-law form dN/dM \propto M^{-\alpha}, with \alpha~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 5 - 15% to the total number of Cha-I members.