A<i>SPITZER</i>c2d LEGACY SURVEY TO IDENTIFY AND CHARACTERIZE DISKS WITH INNER DUST HOLES

Understanding how disks dissipate is essential to studies of planet formation. However, identifying exactly how&#13;\ndust and gas dissipate is complicated due to the difficulty of finding objects that are clearly in the transition phase of&#13;\nlosing their surrounding material.We use Spitzer Infrared Spectrograph (IRS) spectra to examine 35 photometrically&#13;\nselected candidate cold disks (disks with large inner dust holes). The infrared spectra are supplemented with optical&#13;\nspectra to determine stellar and accretion properties and 1.3 mm photometry to measure disk masses. Based on&#13;\ndetailed spectral energy distribution modeling, we identify 15 new cold disks. The remaining 20 objects have&#13;\nIRS spectra that are consistent with disks without holes, disks that are observed close to edge-on, or stars with&#13;\nbackground emission. Based on these results, we determine reliable criteria to identify disks with inner holes from&#13;\nSpitzer photometry, and examine criteria already in the literature. Applying these criteria to the c2d surveyed starforming&#13;\nregions gives a frequency of such objects of at least 4% and most likely of order 12% of the young stellar&#13;\nobject population identified by Spitzer.We also examine the properties of these new cold disks in combination with&#13;\ncold disks from the literature. Hole sizes in this sample are generally smaller than in previously discovered disks&#13;\nand reflect a distribution in better agreement with exoplanet orbit radii. We find correlations between hole size&#13;\nand both disk and stellar masses. Silicate features, including crystalline features, are present in the overwhelming&#13;\nmajority of the sample, although the 10μm feature strength above the continuum declines for holes with radii larger&#13;\nthan ~7 AU. In contrast, polycyclic aromatic hydrocarbons are only detected in 2 out of 15 sources. Only a quarter&#13;\nof the cold disk sample shows no signs of accretion, making it unlikely that photoevaporation is the dominant&#13;\nhole-forming process in most cases.