Planetesimals and protoplanets form from gas and dust that comprise a protoplanetary disk while the disk disperses. After the dispersal, a planetary system/debris disk is left over as a remnant of a protoplanetary disk. Understanding the disk evolution from a gas-rich, optically thick protoplanetary disk to a gas-poor, optically thin debris disk/planetary system is critical to understanding planet formation. However, the overall picture still needs to be clarified and is still an open question.
Recent observations have revealed a group of gas-rich, optically thin disks, so-called hybrid disks. These objects are considered to be at the intermediate point between protoplanetary and debris stages. Therefore, investigating the origin and fate of hybrid disks can give hints to understanding the overall disk evolution.
Theoretically, protoplanetary disks disperse through viscous accretion, magnetohydrodynamics (MHD) winds, and photoevaporation. Viscous accretion and MHD winds dominate mass loss at the early stage, while photoevaporation takes over at the later stage. Hence, photoevaporation likely determines the fate of gas in hybrid disks.
In this study, we perform a suite of radiation hydrodynamics simulations with non-equilibrium thermochemistry. A gas-rich, optically-thin disk is irradiated by the central intermediate-mass star and interstellar radiation field. We measure the mass-loss rates due to photoevaporation to estimate the lifetime of the gas disk. We then compare it with the ages of detected hybrid disks and discuss their origin and fate. We also apply our results to the origin of gas-rich debris disks, which are debris disks but have rich gas content. Two possible scenarios have been proposed to explain the origin of the gas: the primordial-origin scenario tells that the gas is a protoplanetary remnant. In contrast, the secondary-origin scenario attributes it to releasing gas from large colliding bodies. The primordial-origin scenario has yet to be studied in detail. Here, we discuss its plausibility.