EVIDENCE FOR DISK PHOTOEVAPORATION DRIVEN BY THE CENTRAL STAR

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 similar to 30,000) spectra of the [Ne II] 12.81 mu m 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 similar to 6 km s(-1) 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.