Two-photon Production in Low-velocity Shocks

The Galactic interstellar medium abounds in shocks with low velocities v _s ≲ 70 km s⁻¹. Some are descendants of higher velocity shocks, while others start off at low velocity (e.g., stellar bow shocks, intermediate velocity clouds, spiral density waves). Low-velocity shocks cool primarily via Lyα and two-photon continuum, augmented by optical recombination lines (e.g., Hα), forbidden lines of metals and free-bound emission, free–free emission. The dark far-ultraviolet (FUV) sky, aided by the fact that the two-photon continuum peaks at 1400 Å, makes the FUV band an ideal tracer of low-velocity shocks. GALEX FUV images reaffirm this expectation, discovering faint and large interstellar structure in old supernova remnants and thin arcs stretching across the sky. Interstellar bow shocks are expected from fast stars from the Galactic disk passing through the numerous gas clouds in the local interstellar medium within 15 pc of the Sun. Using the bests atomic data available to date, we present convenient fitting formulae for yields of Lyα, two-photon continuum, and Hα for pure hydrogen plasma in the temperature range of 10⁴–10⁵ K. The formulae presented here can be readily incorporated into time-dependent cooling models as well as collisional ionization equilibrium models.