  We present new X-ray and Far Ultraviolet (FUV) observations of T Tauri stars
covering the age range 1 to 10 Myr. Our goals are to observationally constrain
the intensity of radiation fields responsible for evaporating gas from the
circumstellar disk and to assess the feasibility of current photoevaporation
models, focusing on X-ray and UV radiation. We greatly increase the number of
7-10 Myr old T Tauri stars observed in the X-rays by including observations of
the well populated 25 Ori aggregate in the Orion OB1a subassociation. With
these new 7-10 Myr objects, we confirm that X-ray emission remains constant
from 1-10 Myr. We also show, for the first time, observational evidence for the
evolution of FUV radiation fields with a sample of 56 accreting and
non-accreting young stars spanning 1 Myr to 1 Gyr. We find that the FUV
emission decreases on timescales consistent with the decline of accretion in
classical T Tauri stars until reaching the chromospheric level in weak T Tauri
stars and debris disks. Overall, we find that the observed strength of high
energy radiation is consistent with that required by photoevaporation models to
dissipate the disks in timescales of approximately 10 Myr. Finally, we find
that the high energy fields that affect gas evolution are not similarly
affecting dust evolution; in particular, we find that disks with inner
clearings, the transitional disks, have similar levels of FUV emission as full
disks.
