Photoevaporation of Grain-Depleted Protoplanetary Disks around Intermediate-Mass Stars: Investigating Possibility of Gas-Rich Debris Disks as Protoplanetary Remnants

TL;DR

This study models photoevaporation of gas-rich, grain-depleted protoplanetary disks around intermediate-mass stars, finding EUV-driven mass loss can allow gas to survive for tens of millions of years, supporting the idea they are remnants.

Contribution

It demonstrates that grain depletion suppresses FUV photoevaporation, making EUV the dominant process, and shows gas can persist longer around A-type stars, explaining observed gas-rich debris disks.

Findings

EUV photoevaporation dominates in grain-depleted disks.

Gas can survive for ~50 Myr depending on initial mass.

Longer gas lifetimes around A-type stars support the remnant hypothesis.

Abstract

Debris disks are classically considered to be gas-less systems, but recent (sub)millimeter observations have detected tens of those with rich gas content. The origin of the gas component remains unclear; namely, it can be protoplanetary remnants and/or secondary products deriving from large bodies. In order to be protoplanetary in origin, the gas component of the parental protoplanetary disk is required to survive for $\gtrsim10{\,\rm Myr}$. However, previous models predict $\lesssim 10{\,\rm Myr}$ lifetimes because of efficient photoevaporation at the late stage of disk evolution. In the present study, we investigate photoevaporation of gas-rich, optically-thin disks around intermediate-mass stars at a late stage of the disk evolution. The evolved system is modeled as those where radiation force is sufficiently strong to continuously blow out small grains ($\lesssim 4 {\,\rm \mu m}$), which are an essential component for driving photoevaporation via photoelectric heating induced by stellar far-ultraviolet (FUV). We find that the grain depletion reduces photoelectric heating, so that FUV photoevaporation is not excited. Extreme-ultraviolet (EUV) photoevaporation is dominant and yields a mass-loss rate of $2$--$5\times10^{-10}(\Phi_{\rm EUV}/10^{41}{\,\rm s}^{-1})^{1/2}\,M_\odot\,{\rm yr}^{-1}$, where $\Phi_{\rm EUV}$ is the EUV emission rate. The estimated lifetimes of the gas component are $\sim 50 (M_{\rm disk}/10^{-2}\,M_\odot)(\Phi_{\rm EUV}/10^{41}\,{\rm s}^{-1})^{1/2}\,{\rm Myr}$ and depend on the ``initial'' disk mass at the point small grains have been depleted in the system. With an order estimation, we show that the gas component can survive for a much longer time around A-type stars than lower-mass stars. This trend is consistent with the higher frequency of gas-rich debris disks around A-type stars, implying the possibility of the gas component being protoplanetary remnants.