  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 [NeII] 12.81 micron line from 7
circumstellar disks using VISIR on Melipal/VLT. We show that the 3 transition
disks in the sample all have [NeII] line profiles consistent with those
predicted by a photoevaporative flow driven by stellar extreme UV photons. The
~6 km/s blue-shift 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 [NeII] line close to the stellar velocity from the sample of
classical optically thick (non-transition) disks. We conclude that most of the
spectrally unresolved [NeII] emission in these less evolved systems arises from
jets/outflows rather than from the disk. The pattern of the [NeII] detections
and non-detections suggests that extreme UV-driven photoevaporation starts only
at a later stage in the disk evolution.
