The cida variability survey of orion OB1.: I.: The low-mass population of ORI OB1A and 1B

We present results of a large-scale, multiepoch optical survey of the Orion OB1 association, carried out with the QUEST camera at the Venezuela National Astronomical Observatory. We identify for the first time the widely spread low-mass, young population in the Ori OB1a and OB1b subassociations. Candidate members were picked up by their variability in the V band and position in color-magnitude diagrams. We obtained spectra to confirm membership. In a 2 region spanning similar to68 deg(2), we found 197 new young stars; of these, 56 are located in the Ori OB1a subassociation and 141 in Ori OB1b. The spatial distribution of the low-mass young stars is spatially coincident with that of the high-mass members but suggests a much sharper edge to the association. Comparison with the spatial extent of molecular gas and extinction maps indicates that the subassociation Ori OB1b is concentrated within a ringlike structure of radius similar to2degrees(similar to15 pc at 440 pc), centered roughly on the star epsilon Ori in the Orion belt. The ring is apparent in (CO)-C-13 and corresponds to a region with an extinction A(nu) greater than or equal to 1. The stars exhibiting strong Halpha emission, an indicator of active accretion, are found along this ring, whereas the center is populated with weak Ha.-emitting stars. In contrast, Ori OB1a is located in a region devoid of gas and dust. We identify a grouping of stars within a similar to3 deg(2) area located in Ori OB1a, roughly clustered around the B2 star 25 Ori. The Herbig Ae/Be star V346 Ori is also associated with this grouping, which could be an older analog of sigma Ori. Using several sets of evolutionary tracks, we find an age of 710 Myr for Ori OB1a and of similar to4-6 Myr for Ori OB1b, consistent with previous estimates from OB stars. Indicators such as the equivalent width of Halpha and near-IR excesses show that the number of accreting low-mass stars decreases sharply between Ori OB1b and Ori OB1a. These results indicate that although a substantial fraction of accreting disks remain at ages similar to5 Myr, inner disks are essentially dissipated by 10 Myr.