The Cida Variability Survey of Orion Ob1. I: the Low-mass Population of Ori Ob 1a and 1b

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 ∼ 682 we found 197 new young stars; of these, 56 are located in the Ori OB1a subassociation and 142 in Ori OB1b. The spatial distribution of the 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 1b is concentrated within a ring-like structure of radius ∼ 2(∼ 15 pc at 440 pc), centered roughly on the star ǫ 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 Hα emission, an indicator of active accretion, are found along this ring, while the center is populated with weak Hα emitting stars. In contrast, Ori OB1a is located in a region devoid of gas and dust. We identify a grouping of stars Based on observations obtained at the Llano del Hato National Astronomical Observatory of Venezuela, operated by CIDA for the Ministerio de Ciencia y Tecnoloǵıa, and at the Fred Lawrence Whipple Observatory (FLWO) of the Smithsonian Institution. Based on observations obtained at the 3.5m WIYN telescope. The WIYN Observatory is a joint facility of the University of Wisconsin-Madison, Indiana University, Yale University, and the National Optical Astronomy Observatory. Centro de Investigaciones de Astronomı́a (CIDA), Apartado Postal 264, Mérida 5101-A, Venezuela; Electronic mail: [email protected],[email protected],[email protected] Smithsonian Astrophysical Observatory, Mail Stop 42, Cambridge, MA 02138, USA; Electronic mail: [email protected],[email protected],[email protected] Also at the Postgrado de F́ısica Fundamental, Universidad de los Andes, Mérida, Venezuela Also at the Postgrado en F́ısica, Universidad Central de Venezuela, Caracas, Venezuela Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.