We calculate the rate of photoevaporation of a circumstellar disk by energetic radiation ( FUV , 6eV < h \nu < 13.6eV ; EUV , 13.6eV < h \nu < 0.1keV ; and Xrays , h \nu > 0.1 keV ) from its central star .
We focus on the effects of FUV and X-ray photons since EUV photoevaporation has been treated previously , and consider central star masses in the range 0.3 - 7 { M } _ { \odot } .
Contrary to the EUV photoevaporation scenario , which creates a gap at about r _ { g } \sim 7 ( M _ { * } / 1 { M } _ { \odot } ) AU and then erodes the outer disk from inside out , we find that FUV photoevaporation predominantly removes less bound gas from the outer disk .
Heating by FUV photons can cause significant erosion of the outer disk where most of the mass is typically located .
X-rays indirectly increase the mass loss rates ( by a factor \sim 2 ) by ionizing the gas , thereby reducing the positive charge on grains and PAHs and enhancing FUV-induced grain photoelectric heating .
FUV and X-ray photons may create a gap in the disk at \sim 10 AU under favourable circumstances .
Photoevaporation timescales for M _ { * } \sim 1 { M } _ { \odot } stars are estimated to be \sim 10 ^ { 6 } years , after the onset of disk irradiation by FUV and X-rays .
Disk lifetimes do not vary much for stellar masses in the range 0.3 - 3 M _ { \odot } .
More massive stars ( \gtrsim 7 { M } _ { \odot } ) lose their disks rapidly ( in \sim 10 ^ { 5 } years ) due to their high EUV and FUV fields .
Disk lifetimes are shorter for shallow surface density distributions and when the dust opacity in the disk is reduced by processes such as grain growth or settling .
The latter suggests that the photoevaporation process may accelerate as the dust disk evolves .