Disk Evolution in Cep OB2: Results from theSpitzer Space Telescope

We present the results of an infrared imaging survey of two clusters in the Cep OB2 Association, Tr 37 and NGC 7160, using the IRAC and MIPS instruments on board the Spitzer Space Telescope. Our observations cover the wavelengthrangefrom3.6to24 m,allowingustodetectdiskemissionoveratypicalrangeofradii 0.1to 20AU from the central star. In Tr 37, with an age of about 4 Myr, about 48% of the low-mass stars exhibit detectable disk emission in the IRAC bands. Roughly 10% of the stars with disks may be ‘‘transition’’ objects, with essentially photospheric fluxes at wavelengths 4.5 m but with excesses at longer wavelengths, indicating an optically thin inner disk. The median optically thick disk emission in Tr 37 is lower than the corresponding median for stars in the youngerTaurusregion;thedecreaseininfraredexcessislargerat6–8 mthanat24 m,suggestingthatgraingrowth and/or dust settling has proceeded faster at smaller disk radii, as expected on general theoretical grounds. Only about 4% of the low-mass stars in the 10 Myr old cluster NGC 7160 show detectable infrared disk emission. We also find evidence for 24 m excesses around a few intermediate-mass stars, which may represent so-called ‘‘debris disk’’ systems. Our observations provide new constraints on disk evolution through an important age range. Subject headingg accretion, accretion disks — planetary systems: protoplanetary disks — stars: pre–main-sequence