2010ApJ...708.1107M

Protoplanetary disks with AU-scale inner clearings, often referred to as transitional disks, provide a unique sample for understanding disk dissipation mechanisms and possible connections to planet formation. Observations of young stellar clusters with the Spitzer Space Telescope have amassed mid-infrared (IR) spectral energy distributions (SEDs) for thousands of star-disk systems from which transition disks can be identified. From a sample of eight relatively nearby young regions (d ≲ 400 pc), we have identified about 20 such objects, which we term "classical" transition disks, spanning a wide range of stellar age and mass. We employed strict IR continuum criteria to limit ambiguity: an 8-24 µm spectral slope limit (α>0) to select for robust optically thick outer disks, and 3.6-5.8 µm spectral slope and 5.8 µm continuum excess limits to select for optically thin or zero continuum excess from the inner few AU of the disks. We also identified two additional categories representing more ambiguous cases: "warm excess" objects with transition-like SEDs but moderate excess at 5.8 µm, and "weak excess" objects with smaller 24 µm excess that may be optically thin or exhibit advanced dust grain growth and settling. From existing Hα emission measurements, we find evidence for different accretion activity among the three categories, with a majority of the classical and warm excess transition objects still accreting gas through their inner holes and onto the central stars, while a smaller fraction of the weak transition objects are accreting at detectable rates. We find a possible age dependence on the frequency of classical transition objects, with fractions relative to the total population of disks in a given region of a few percent at 1-2 Myr rising to 10%-20% at 3-10 Myr. The trend is even stronger if the weak and warm excess objects are included. This relationship may be due to a dependence of the outer disk clearing timescale with stellar age, suggesting a variety of clearing mechanisms working at different times, or it may reflect that a smaller fraction of all disks actually undergo an inner clearing phase at younger ages. Classical transition disks appear to be less common, and weak transition disks more common, around lower-mass stars (M ≲ 0.3 M_☉), which we suggest may be a further indicator of the stellar mass-dependent disk evolution that has been seen in previous studies. The difference in number statistics and accretion activity between the two classes further suggests that they are not connected but rather represent distinct evolutionary outcomes for disks.