  We present results of a large scale, multi-epoch optical survey of the Ori
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 Orion OB1a and OB1b sub-associations.
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 region spanning ~ 68 deg^2 we found 197 new young stars; of
these, 56 are located in the Ori OB1a subassociation and 142 in Ori OB1b.
Comparison with the spatial extent of molecular gas and extinction maps
indicates that the subassociation Ori 1b is concentrated within a ring-like
structure of radius ~2 deg (~15 pc at 440 pc), centered roughly on the star
epsilon Ori in the Orion belt. The ring is apparent in 13CO and corresponds to
a region with an extinction Av>=1. The stars exhibiting strong Ha emission, an
indicator of active accretion, are found along this ring, while 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 ~3
deg^2 area located in 1a, 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 using several sets of evolutionary
tracks 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 the
equivalent width of Ha and near-IR excesses show that while a substantial
fraction of accreting disks remain at ages ~5 Myr, inner disks are essentially
dissipated by 10 Myr.

Astrophysics