Circumstellar Disk Candidates Identified from Ultraviolet Excesses in the Orion Nebula Cluster Flanking Fields

Abstract

We present an optical study of four 45@ ] 45@ Ðelds centered east, west, north, and south of the D0¡ .5 Orion Nebula Cluster center. We have measured V and photometry for D5000 stars, and U photo-I C metry for D1600 of these. We have obtained spectral classiÐcations for D300 of the stars with UV I C photometry plus an additional D200 stars located outside the area of our photometric survey. Dered-dened photometry allows us to investigate the evidence for circumstellar accretion disks from excess emission at ultraviolet wavelengths, as well as the mass and age distributions of our sample. We Ðnd D230 active accretion disk candidates and estimate the accretion disk fraction at We use the Z40%. magnitude of the ultraviolet excess to infer disk accretion rates. The mass accretion rate decreases (M 0) slowly with age over the limited age range spanned by our data, 1È3 Myr, and increases slowly with mass over the limited mass range 0.2È1.2 Although complicated selection e ects are likely to a ect M _ . the overall trends with mass and age, we are sensitive to, but do not see, high associated with M 0 -values the older ages and lower masses in our sample. The mean value of is D10~8 yr~1, with a range M 0 M _ of more than 1 order of magnitude at all ages and masses. We Ðnd an age and age spread for our sample of 106.0B0.4 yr with no variation within or between our Ðelds. Meaningful constraint of the mass distribution is precluded because of the completeness limits of our survey.