A large-scale optical-near-infrared survey for brown dwarfs and very low mass stars in the orion OB1 association

We report the initial results of a large-scale optical-near-infrared (IR) survey to extend the known young population of the entire Orion star-forming region down to the substellar domain. Using deep optical I-band photometry and data from the Two Micron All Sky Survey (2MASS), we selected candidates across 14.8 deg² in the 8 Myr old Ori OB1a subassociation and over 6.7 deg² in the 3 Myr old Ori OB1b subassociation, with completeness down to 0.05 M and 0.072 M respectively. We obtained low-resolution optical spectra for a subsample of four candidates in Ori OB1a and 26 in Ori OB1b; as a result we confirmed three new members in Ori OB1a, one of which is substellar, and 19 new members in Ori OB1b, out of which seven are at the substellar limit and five are substellar. We looked into the presence of accretion signatures by measuring the strength of the Hα emission line. Accordingly, we classified the new members as having Classical T-Tauri star-like (CTTS) or Weak Lined T Tauri star-like (WTTS) nature. We found that all the new members confirmed in Ori OB1a are WTTSs, while 39⁺²⁵ _-22% of the new members in Ori OB1b exhibit CTTS-like behavior, suggestive of ongoing accretion from a circum(sub)stellar disk. Additionally we found that none of the members confirmed in OB1a shows near-IR color excess while 38⁺²⁶ _-21% of OB1b members show H - K color excess. These results are consistent with recent findings for low-mass young stars in Orion OB1. The similarity in CTTS-like properties and near-IR excess across the substellar boundary gives support to the idea of a common formation mechanism for low-mass stars and at least the most massive brown dwarfs. Finally, we comment on the discovery of two new members classified as CTTSs, both exhibiting W(Hα) ≲ -140 , suggesting significant ongoing accretion.