The hydroxyl satellite-line 'flip' as a tracer of expanding H II regions

Observations of the four ²Π_3/2, J = 3/2 ground state transitions of the hydroxyl radical (OH) have emerged as an informative tracer of molecular gas in the Galactic interstellar medium (ISM). We discuss an OH spectral feature known as the 'flip', in which the satellite lines at 1612 and 1720 MHz flip - one from emission to absorption and the other the reverse - across a closely blended double feature. We highlight 30 examples of the flip from the literature, 27 of which exhibit the same orientation with respect to velocity: the 1720-MHz line is seen in emission at more negative velocities. These same examples are also observed towards bright background continuum, many (perhaps all) show stimulated emission, and 23 of these are coincident in on-sky position and velocity with H ii radio recombination lines. To explain these remarkable correlations, we propose that the 1720-MHz stimulated emission originates in heated and compressed post-shock gas expanding away from a central H ii region, which collides with cooler and more diffuse gas hosting the 1612-MHz stimulated emission. The foreground gas dominates the spectrum due to the bright central continuum; hence, the expanding post-shock gas is blue-shifted relative to the stationary pre-shock gas. We employ non-local thermodynamic equilibrium (LTE) excitation modelling to examine this scenario and find that indeed FIR emission from warm dust adjacent to the H ii region radiatively pumps the 1612-MHz line in the diffuse, cool gas ahead of the expanding shock front, while collisional pumping in the warm, dense shocked gas inverts the 1720-MHz line.