  The structure of static atmospheres around unmagnetized neutron stars
undergoing steady, spherical accretion is discussed. We focus on the ``hot''
configurations presented by Turolla et al. (1994) and calculate the radiation
spectrum using a characteristics method. In particular, it is found that
e$^+$--e$^-$ pair production may affect significantly the external atmospheric
layers, where positron and proton number densities become of the same order.
The consequent increase of the scattering opacity lowers the Eddington limit
and this, in turn, may drive a dynamical instability if the accretion
luminosity is large enough, ultimately producing a rapid expulsion of the
envelope. If ``hot'' states are indeed accessible, this mechanism could give
rise to transient phenomena in hard X--rays of potential great astrophysical
interest.
