  Photoionization heating from UV radiation incident on the atmospheres of hot
Jupiters may drive planetary mass loss. We construct a model of escape that
includes realistic heating and cooling, ionization balance, tidal gravity, and
pressure confinement by the host star wind. We show that mass loss takes the
form of a hydrodynamic ("Parker") wind, emitted from the planet's dayside
during lulls in the stellar wind. When dayside winds are suppressed by the
confining action of the stellar wind, nightside winds might pick up if there is
sufficient horizontal transport of heat. A hot Jupiter loses mass at maximum
rates of ~2 x 10^12 g/s during its host star's pre-main-sequence phase and ~2
x10^10 g/s during the star's main sequence lifetime, for total maximum losses
of ~0.06% and ~0.6% of the planet's mass, respectively. For UV fluxes F_UV <
10^4 erg/cm^2/s, the mass loss rate is approximately energy-limited and is
proportional to F_UV^0.9. For larger UV fluxes, such as those typical of T
Tauri stars, radiative losses and plasma recombination force the mass loss rate
to increase more slowly as F_UV^0.6. Dayside winds are quenched during the T
Tauri phase because of confinement by overwhelming stellar wind pressure. We
conclude that while UV radiation can indeed drive winds from hot Jupiters, such
winds cannot significantly alter planetary masses during any evolutionary
stage. They can, however, produce observable signatures. Candidates for
explaining why the Lyman-alpha photons of HD 209458 are absorbed at
Doppler-shifted velocities of +/- 100 km/s include charge-exchange in the shock
between the planetary and stellar winds.
