Astron. J., 129, 907-926 (2005/February-0)
The CIDA variability survey of Orion OB1. I. The low-mass population of Ori OB1a and 1b.
BRICENO C., CALVET N., HERNANDEZ J., VIVAS A.K., HARTMANN L., DOWNES J.J. and BERLIND P.
Abstract (from CDS):
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 region spanning ∼68 deg2, 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 ∼2°(∼15 pc at 440 pc), centered roughly on the star ε 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, whereas 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 within a ∼3 deg2 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 σ Ori. Using several sets of evolutionary tracks, we find an age of 7-10 Myr for Ori OB1a and of ∼4-6 Myr for Ori OB1b, consistent with previous estimates from OB stars. Indicators such as the equivalent width of Hα 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 ∼5 Myr, inner disks are essentially dissipated by 10 Myr.
Stars: Formation - Stars: Pre-Main-Sequence - Surveys
VizieR on-line data:
<Available at CDS (J/AJ/129/907): table3.dat table4.dat table6.dat table7.dat>
Table 3: CVSO NNN (Nos 1-197).
Grouping of stars around 25 Ori mentioned in abstract later dubbed NAME 25 Ori GROUP.
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