We report the initial results of a large-scale optical-near infrared 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 2MASS survey , we selected candidates across \sim 14.8 deg ^ { 2 } in the \sim 8 Myr old Ori OB1a subassociation and over \sim 6.7 deg ^ { 2 } in the Ori OB1b subassociation ( age \sim 3 Myr ) , with completeness down to 0.05 M _ { \odot } and 0.072 M _ { \odot } respectively .
We obtained low resolution optical spectra for a subsample of 4 candidates in Ori OB1a and 26 in Ori OB1b ; as a result we confirmed 3 new members in Ori OB1a , one of which is substellar , and 19 new members in Ori OB1b , out of which 7 are at the substellar limit and 5 are substellar .
We looked into the presence of accretion signatures by measuring the strength of the H \alpha line in emission .
Accordingly , we classified the new members as having Classical T-Tauri star ( 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 show 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 remark the discovery of two new members classified as CTTSs , both exhibiting W ( H \alpha ) \lesssim - 140 Å , suggesting significant ongoing accretion .