The CIDA Variability Survey of Orion OB1. I. The Low-Mass Population of Ori OB1a and 1b

We present results of a large scale, multi-epoch optical survey of the OriOB1 association, carried out with the QuEST camera at the Venezuela NationalAstronomical Observatory. We identify for the first time the widely spreadlow-mass, young population in the Orion OB1a and OB1b sub-associations.Candidate members were picked up by their variability in the V-band andposition in color-magnitude diagrams. We obtained spectra to confirmmembership. In a region spanning ~ 68 deg^2 we found 197 new young stars; ofthese, 56 are located in the Ori OB1a subassociation and 142 in Ori OB1b.Comparison with the spatial extent of molecular gas and extinction mapsindicates that the subassociation Ori 1b is concentrated within a ring-likestructure of radius ~2 deg (~15 pc at 440 pc), centered roughly on the starepsilon Ori in the Orion belt. The ring is apparent in 13CO and corresponds toa region with an extinction Av>=1. The stars exhibiting strong Ha emission, anindicator of active accretion, are found along this ring, while the center ispopulated with weak Ha emitting stars. In contrast, Ori OB1a is located in aregion devoid of gas and dust. We identify a grouping of stars within a ~3deg^2 area located in 1a, roughly clustered around the B2 star 25 Ori. TheHerbig Ae/Be star V346 Ori is also associated with this grouping, which couldbe an older analog of sigma Ori. Using using several sets of evolutionarytracks we find an age of 7 - 10 Myr for Ori 1a and of ~4 - 6 Myr for Ori OB1b,consistent with previous estimates from OB stars. Indicators such as theequivalent width of Ha and near-IR excesses show that while a substantialfraction of accreting disks remain at ages ~5 Myr, inner disks are essentiallydissipated by 10 Myr.