Photoevaporation of Grain-depleted Protoplanetary Disks around Intermediate-mass Stars: Investigating the Possibility of Gas-rich Debris Disks as Protoplanetary Remnants

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