Cepheus OB2: Disk Evolution and Accretion at 3-10 Myr

Abstract

We present the results of MMT observations of young stars for our study of protoplanetary disks at ages 1-10 Myr in two young clusters located in the Cepheus OB2 region: Trumpler 37 (embedded in the H II region IC 1396) and NGC 7160. Using low-resolution optical spectra from the Hectospec multifiber spectrograph, we have tripled the number of known low-mass cluster members, identifying ~130 new members in Tr 37 and ~30 in NGC 7160. We use indicators of youth (Li absorption at 6707 Å) and accretion/chromospheric activity (Hα emission) to identify and classify the low-mass cluster members. We derive spectral types for all the low-mass candidates and calculate the individual extinctions and the average over the clusters. With the extended member samples, we estimate the disk fraction in the clusters, finding that ~40% of the low-mass stars in Tr 37 are actively accreting, whereas only 1 of the ~55 low-mass stars in NGC 7160 shows indications of accretion. Optical photometry and theoretical isochrones are used to determine the age of the cluster members, confirming the estimates of ~4 Myr for Tr 37 and ~10 Myr for NGC 7160. Accretion rates in Tr 37 (~10-8Msolaryr-1 on average) are derived from U-band photometry. We find that only ~50% of the accreting stars have near-IR excesses (from 2MASS), which could be due to the geometry of their disks or be an indication dust of settling/grain growth. Finally, we study the high- and intermediate-mass members of the clusters. With the extended member list, we revise the spatial distribution of stars with disks. Our results are crucial for interpreting Spitzer Space Telescope studies of accretion disks at the ages of planet formation (3-10 Myr). Observations reported here were obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona.