  We present new models for the X-ray photoevaporation of circumstellar discs
which suggest that the resulting mass loss (occurring mainly over the radial
range 10-40 AU) may be the dominant dispersal mechanism for gas around low mass
pre-main sequence stars, contrary to the conclusions of previous workers. Our
models combine use of the MOCASSIN Monte Carlo radiative transfer code and a
self-consistent solution of the hydrostatic structure of the irradiated disc.
We estimate the resulting photoevaporation rates assuming sonic outflow at the
surface where the gas temperature equals the local escape temperature and
derive mass loss rates of ~10^{-9} M_sun/yr, typically a factor 2-10 times
lower than the corresponding rates in our previous work (Ercolano et al., 2008)
where we did not adjust the density structure of the irradiated disc. The
somewhat lower rates, and the fact that mass loss is concentrated towards
slightly smaller radii, result from the puffing up of the heated disc at a few
AU which partially screens the disc at tens of AU. (.....) We highlight the
fact that X-ray photoevaporation has two generic advantages for disc dispersal
compared with photoevaporation by extreme ultraviolet (EUV) photons that are
only modestly beyond the Lyman limit: the demonstrably large X-ray fluxes of
young stars even after they have lost their discs and the fact that X-rays are
effective at penetrating much larger columns of material close to the star
(abridged).
