  Direct and statistical observational evidences suggest that photoevaporation
is important in eroding the atmosphere of sub-Neptune planets. We construct
full hydrodynamic simulations, coupled with consistent thermochemistry and
ray-tracing radiative transfer, to understand the physics of atmospheric
photoevaporation caused by high energy photons from the host star. We identify
a region on the parameter space where a hydrostatic atmosphere cannot be
balanced by any plausible interplanetary pressure, so that the atmosphere is
particularly susceptible to loss by Parker wind. This region may lead an
absence of rich atmosphere (substantially H/He) for planets with low mass (M ~
3 M_earth). Improving on previous works, our simulations include detailed
microphysics and a self-consistent thermochemical network. Full numerical
simulations of photoevaporative outflows shows a typical outflow speed ~ 30
km/s and Mdot ~ 4e-10 M_earth/yr for a 5 M_earth fiducial model rocky-core
planet with 1e-2 of its mass in the atmosphere. Supersonic outflows are not
quenched by stellar wind ram pressure (up to 5 times the total pressure at
transonic points of the fiducial model). The outflows modulated by stellar wind
are collimated towards the night side of the planet, while the mass loss rate
is only ~ 25% lower than the fiducial model. By exploring the parameter space,
we find that EUV photoionization is most important in launching
photoevaporative wind. Other energetic radiation, including X-ray, are of
secondary importance. The leading cooling mechanism is ro-vibrational molecular
cooling and adiabatic expansion rather than recombination or Ly alpha cooling.
The wind speed is considerably higher than the escape velocity at the wind base
in most cases, hence the mass loss rate is proportional to the second power of
the EUV photosphere size R_euv, instead of the third, as suggested by previous
works...
