From Classical Disks to Transition Disks: An Increasing Dust-to-Gas Ratio?

Transition disks are a subgroup of protoplanetary disks with comparatively small near-IR excesses, but large mid/far-IR excesses. These IR excesses are characteristic of an inner, optically thin dust cavity, extending ~AU from the central star. The existence of dust gaps within transition disks has been confirmed by high-resolution continuum observations at millimeter wavelengths. It is widely hypothesized that the formation of dust gaps is an intermediate process in the eventual dispersal of the entire protoplanetary disk. Here, we compare the far-IR properties of 25 transition disks to a comparable sample of classical protoplanetary disks. We report new Herschel/PACS spectroscopy of the [OI] 63 μm emission line and adjacent continuum. The [OI] 63 μm line traces cool (