Self-generated ultraviolet radiation in molecular shock waves I. Effects
  of Lyman $\alpha$, Lyman $\beta$, and two-photon continuum
  Shocks are ubiquitous in the interstellar and intergalactic media, where
their chemical and radiative signatures reveal the physical conditions in which
they arise. Detailed astrochemical models of shocks at all velocities are
necessary to understand the physics of many environments including protostellar
outflows, supernova remnants, and galactic outflows.
  We present an accurate treatment of the self-generated UV radiation in
intermediate velocity, stationary, weakly magnetised, J-type, molecular shocks.
Shock solutions computed with the Paris-Durham shock code are post-processed
using a multi-level accelerated $\Lambda$-iteration radiative transfer
algorithm to compute Ly$\alpha$, Ly$\beta$, and 2-photon continuum emission.
The subsequent impacts on the ionisation and dissociation of key atomic and
molecular species as well as on the heating by the photoelectric effect take
the wavelength dependent cross-sections and the fluid velocity profile into
account. We analyse shock models with velocities $V=25-60$ km/s, propagating in
dense ($n \geq 10^4$ ${\rm cm}^{-3}$), shielded gas.
  Self-absorption traps Ly$\alpha$ photons in a small region in the shock,
though a large fraction escapes into the line wings. We find a critical
velocity $V\sim 30$ km/s above which shocks produce a Ly$\alpha$ photon flux
exceeding that of the standard ISRF. The escaping photons generate a warm slab
(T~100 K) ahead of the shock as well as pre-ionise C and S. These shocks are
traced by bright atomic fine structure (e.g. O and S) and metastable (e.g. O
and C) lines, substantive molecular emission (e.g. H2, OH, and CO), enhanced
column densities of several species (e.g. CH+ and HCO+), as well as a severe
destruction of H2O. As much as 13-21% of the initial kinetic energy of the
shock escapes in Ly$\alpha$ and Ly$\beta$ photons if the dust opacity in the
radiative precursor allows it.
