Photoevaporation from Inner Protoplanetary Disks Confronted with Observations

Abstract

The decades-long explorations on the dispersal of protoplanetary disks involve many debates about photoevaporation versus magnetized wind launching mechanisms. This letter argues that the observed winds originating from the inner disk ($R\lesssim 0.3$ AU) cannot be explained by the photoevaporative mechanism. Energy conservation requires the presumed photoevaporative winds to be heated to $\gtrsim 10^5$ K when launched from inner disks. However, due to efficient thermal accommodation with dust grains and cooling processes at high densities, X-ray irradiation at energies above 1 keV cannot efficiently launch winds in the first place because of its high penetration. Some studies claiming X-ray wind launching have oversimplified the thermochemical couplings. Furthermore, heating the gas to escape velocity will over-ionize it, suppressing the species responsible for observed forbidden lines (e.g., [OI] 6300 $\r{A}$ ). Confirmed by semi-analytic integrations of thermochemical fluid structures, such high ionizations contradict the observed emission of neutral and singly-ionized atoms from the winds originating from the inner disks.