Abstract
Understanding how circumstellar disks of young stellar objects dissipate gas and small dust grains and evolve into debris disks is crucial for advancing our knowledge in planet formation. Recent observations suggest that multiple pathways may exist for the disk evolution. In order to clearly identify the evolutionary paths by variation of physical parameters, here we perform a statistical analysis on two morphological parameters of the spectral energy distributions (SEDs), λturn-off and αexcess, where λturn-off is the wavelength where the disk flux equal to the stellar flux, and αexcess is the spectral slope at wavelengths longer than λturn-off. We compare λturn-off and αexcess from the theoretical SEDs by Robitaille et al. (2006) with those of the observational results from the Spitzer's c2d Legacy project, and found the distribution of transition disks in disk mass and disk inner radius space. The distribution suggests there are at least two independent evolutionary paths, consistent with, respectively, the typical transition disk scenario and homologously depleted disk scenario. In addition, we find that the Chamaeleon II and Lupus clouds lack of typical transition disks, unlike Ophiuchus, Perseus or Serpens clouds. The difference may be explained by a shorter evolutionary timescale for typical transition disks than that for homologously depleted disks.