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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 beprotoplanetary remnants and/or secondary products deriving from large bodies.In order to be protoplanetary in origin, the gas component of the parentalprotoplanetary disk is required to survive for $\gtrsim10{\,\rm Myr}$. However,previous models predict $\lesssim 10{\,\rm Myr}$ lifetimes because of efficientphotoevaporation at the late stage of disk evolution. In the present study, weinvestigate photoevaporation of gas-rich, optically-thin disks aroundintermediate-mass stars at a late stage of the disk evolution. The evolvedsystem is modeled as those where radiation force is sufficiently strong tocontinuously blow out small grains ($\lesssim 4 {\,\rm \mu m}$), which are anessential component for driving photoevaporation via photoelectric heatinginduced by stellar far-ultraviolet (FUV). We find that the grain depletionreduces photoelectric heating, so that FUV photoevaporation is not excited.Extreme-ultraviolet (EUV) photoevaporation is dominant and yields a mass-lossrate of $2$--$5\times10^{-10}(\Phi_{\rm EUV}/10^{41}{\,\rms}^{-1})^{1/2}\,M_\odot\,{\rm yr}^{-1}$, where $\Phi_{\rm EUV}$ is the EUVemission rate. The estimated lifetimes of the gas component are $\sim 50(M_{\rm disk}/10^{-2}\,M_\odot)(\Phi_{\rm EUV}/10^{41}\,{\rms}^{-1})^{1/2}\,{\rm Myr}$ and depend on the ``initial'' disk mass at the pointsmall grains have been depleted in the system. With an order estimation, weshow that the gas component can survive for a much longer time around A-typestars than lower-mass stars. This trend is consistent with the higher frequencyof gas-rich debris disks around A-type stars, implying the possibility of thegas component being protoplanetary remnants.

astrophysical journal/the astrophysical journal

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