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

Juan José Downes W. (UCV/CIDA), César Briceño Avila (CIDA), Jesús Hernández (Dept. of Astronomy, University of Michigan/CIDA), Nuria Calvet & Lee Hartmann (Dept. of Astronomy, University of Michigan)

Abstract

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 ~14.8 deg2 in the ~10 Myr old Ori OB1a subassociation and over ~6.7 deg2 in the Ori OB1b subassociation (age ~5 Myr), with completeness down to 0.05Mo and 0.072Mo 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 Ha line in emission. Accordingly, we classified the new members as having Classical T-Tauri (CTTS) or Weak T Tauri-like (WTTS) nature. We found that all the new members confirmed in Ori OB1a are WTTS, while 50+/-35% of the substellar members and 28+/-14% of the stellar members in Ori OB1b exhibit CTTS-like behavior, suggestive of ongoing accretion from a circum(sub)stellar disk. These results are consistent with recent findings for low mass young stars in Orion OB1. The similarity in CTTS-like properties across the substellar boundary gives supports 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 CTTS, both exhibiting W(Ha) <= -140A, suggesting significant ongoing accretion.

-- CesarBriceno - 04 Oct 2007