The transition from protoplanetary to debris disks is an important phase of disk evolution. However, the ``transition disks'' identified to date don't fit this description. While their central regions are significantly cleared, the rest of these disks still exhibit small dust grains, large far-infrared excesses, and large gas masses like protoplanetary disks. Most debris disks have central holes, so this characteristic by itself does not define disks transitioning between the two phases.
The debris disk community uses the ratio of infrared excess luminosity to stellar luminosity to characterize a disk's evolutionary state. Unfortunately the young stellar object community uses the slope of the infrared spectral energy distribution to categorize protoplanetary disks, and this parameter is not easily related to the fractional infrared luminosity. As a result, there is no unified parameterization of disk evolutionary state across the crucial boundary between protoplanetary and debris disks.
The fractional infrared luminosities of protoplanetary disks are typically ~10%, whereas for young debris disks such as beta Pictoris it is just ~0.1%. Fractional infrared luminosities near 1% therefore represent disks in transition from the protoplanetary to the debris disk phase, and we refer to them as Luminosity Transition Disks (LTDs).
We present an inventory of known LTDs from the literature. A prototype of the class is HD 141569A, which shows a complex internal structure including rings, gaps, and spirals along with detectable gas emission. The majority of known LTDs are found around early-type stars, implying that a significant population of LTDs remains to be discovered around young low-mass stars.
As the disks in their final clearing stage, LTDs should be ideal targets for imaging planet searches, studying disk structures driven by planetary perturbations, and tracing the dispersal of disk gas via atomic and molecular tracers.