2009ApJ...702..724P

The lifetime of isolated protoplanetary disks is thought to be set by the combination of viscous accretion and photoevaporation driven by stellar high-energy photons. Observational evidence for magnetospheric accretion in young Sun-like stars is robust. Here we report the first observational evidence for disk photoevaporation driven by the central star. We acquired high-resolution (R∼ 30,000) spectra of the [Ne II] 12.81 µm line from seven circumstellar disks using VISIR on Melipal/VLT. We show that the three transition disks in the sample all have [Ne II] line profiles consistent with those predicted by a photoevaporative flow driven by stellar extreme-ultraviolet (EUV) photons. The ∼6 km/s blueshift of the line from the almost face-on disk of TW Hya is clearly inconsistent with emission from a static disk atmosphere and convincingly points to the presence of a photoevaporative wind. We do not detect any [Ne II] line close to the stellar velocity from the sample of classical optically thick (nontransition) disks. We conclude that most of the spectrally unresolved [Ne II] emission in these less-evolved systems arises from jets/outflows rather than from the disk. The pattern of the [Ne II] detections and nondetections suggests that EUV-driven photoevaporation starts only at a later stage in the disk evolution.