2 3 M ay 2 00 9 The Last Gasp of Gas Giant Planet Formation : A Spitzer Study of the 5 Myr - old Cluster NGC 2362

Expanding upon the IRAC survey from Dahm and Hillenbrand (2007), we describe Spitzer IRAC and MIPS observations of the populous, 5 Myr-old open cluster NGC 2362. We analyze the mid-IR colors of cluster members and compared their spectral energy distributions to star+circumstellar disk models to constrain the disk morphologies and evolutionary states. Early/intermediate-type confirmed/candidate cluster members either have photospheric mid-IR emission or weak, optically-thin infrared excess emission at λ ≥ 24 μm consistent with debris disks. Few late-type, solar/subsolar-mass stars have primordial disks. The disk population around late-type stars is dominated by disks with inner holes (canonical ’transition disks’) and ’homologously depleted’ disks. Both types of disks represent an intermediate stage between primordial disks and debris disks. Thus, in agreement with previous results, we find that multiple paths for the primordialto-debris disk transition exist. Because these ’evolved primordial disks’ greatly outnumber primordial disks, our results undermine standard arguments in favor of a . 10 year timescale for the transition based on data from Taurus-Auriga. Because the typical transition timescale is far longer than 10 yr, these data also appear to rule out standard UV photoevaporation scenarios as the primary mechanism to explain the transition. Combining our data with other Spitzer surveys, we investigate the evolution of debris disks around high/intermediatemass stars and investigate timescales for giant planet formation. Consistent with Currie et al. (2008a), the luminosity of 24 μm emission in debris disks due to planet formation peaks at ≈ 10–20 Myr. If the gas and dust in disks evolve Harvard-Smithsonian Center for Astrophysics, 60 Garden St. Cambridge, MA 02140 NASA Exoplanet Science Institute, California Institute of Technology Institute of Astronomy, University of Cambridge, Cambridge, UK