Substellar Objects in Nearby Young Clusters (SONYC). VIII. Substellar population in Lupus 3

Muzic, Koraljka; Scholz, Alexander; Geers, Vincent C.; Jayawardhana, Ray; Lopez Marti, Belen
The Astrophysical Journal, Volume 785, Issue 2, article id. 159, 21 pp. (2014).


SONYC—Substellar Objects in Nearby Young Clusters—is a survey program to investigate the frequency and properties of substellar objects in nearby star-forming regions. We present a new imaging and spectroscopic survey conducted in the young (~1 Myr), nearby (~200 pc) star-forming region Lupus 3. Deep optical and near-infrared images were obtained with MOSAIC-II and NEWFIRM at the CTIO 4 m telescope, covering ~1.4 deg2 on the sky. The i-band completeness limit of 20.3 mag is equivalent to 0.009-0.02 M , for AV <= 5. Photometry and 11-12 yr baseline proper motions were used to select candidate low-mass members of Lupus 3. We performed a spectroscopic follow-up of 123 candidates, using VIMOS at the Very Large Telescope, and we identify 7 probable members, among which 4 have spectral type later than M6.0 and T eff <= 3000 K, i.e., are probably substellar in nature. Two of the new probable members of Lupus 3 appear underluminous for their spectral class and exhibit emission line spectrum with strong Hα or forbidden lines associated with active accretion. We derive a relation between the spectral type and effective temperature: T eff = (4120 ± 175) – (172 ± 26) × SpT, where SpT refers to the M spectral subtype between 1 and 9. Combining our results with the previous works on Lupus 3, we show that the spectral type distribution is consistent with that in other star-forming regions, as well as the derived star-to-brown dwarf ratio of 2.0-3.3. We compile a census of all spectroscopically confirmed low-mass members with spectral type M0 or later.

Based on observations collected at the European Southern Observatory under programs 087.C-0386 and 089.C-0432, and Cerro Tololo Inter-American Observatory's programs 2010A-0054 and 2011A-0144.