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

SONYC – Substellar Objects in Nearby Young Clusters – is a survey programme 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-4m tele-scope, 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 spectroscopic follow-up of 123 candidates, using VIMOS at the Very Large Telescope (VLT), and identify 7 probable members, among which 4 have spectral type later than M6.0 and Teff ≤ 3000 K, i.e. are probably substellar in nature. Two of the new prob-able members of Lupus 3 appear underluminous for their spectral class and exhibit emission line spectrum with strong Hα or forbidden lines associated with active ac-cretion. We derive a relation between the spectral type and effective temperature