2021ApJ...915...90N

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, but it could be protoplanetary remnants and/or secondary products from large bodies. In order to be protoplanetary in origin, the gas component of the parental protoplanetary disk is required to survive for ≳10Myr. However, previous models predict ≲10Myr lifetimes because of efficient photoevaporation at the late stage of disk evolution. 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 like those devoid of small grains (≲4µm). We find that grain depletion reduces photoelectric heating so that far-ultraviolet photoevaporation is not excited. Extreme-ultraviolet (EUV) photoevaporation is dominant and yields a mass-loss rate of the order of 1×10^–11(Φ_EUV/10³⁸s^–1)^1/2M_☉yr^–1, where Φ_EUV is the EUV emission rate of the host star. The estimated gas-disk lifetimes are ∼100(M_disk/10^–3M_☉)(Φ_EUV/10³⁸s^–1)^1/2Myr and depend on the "initial" disk mass at the point small grains have been depleted in the system. We show that the gas component can survive for a much longer time around A-type stars than lower-mass (F-, G-, K-type) stars owing to their atypical low EUV (and X-ray) luminosities. 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.