Dust Clearing in Protoplanetary Disks
ABSTRACT The phase of star formation leading from dust enshrouded star to complete planetary system remains surrounded by mystery. Examples of such transitional state objects in the midst of depleting their dust through accretion and dispersal are rare. However, Spitzer Space Telescope, with its unique wavelength coverage and increased sensitivity, has revealed a new population of such disks. We have obtained Spitzer IRS spectra of a set of disks, including LkHa 330, T Cha, HD 135344 and SR 21, which show a lack of dust emission at 10 microns. Due to the large numbers of circumstellar disks in the 'c2d' Spitzer Legacy Project, a comparison of these unusual disks can be made with non-transitional objects, revealing differences not only in SED shape but potentially in silicate and PAH emission. Detailed modeling of the SEDs attempts to trace the SED back to the physical conditions in the disk. We have also assembled a large amount of complementary data, including Keck Nirspec high-resolution spectra of CO, to test the physical processes active in the disks.