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

Muzic, Koraljka; Scholz, Alexander; Geers, Vincent C.; Jayawardhana, Ray
The Astrophysical Journal, Volume 810, Issue 2, article id. 159, 16 pp. (2015).


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 (\gt 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 {A}V≤slant 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}-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.

Based on observations collected at the European Southern Observatory under the program 093.C-0050(A).