Flips in the Interstellar Medium
Anita Petzler(Macquarie University)
Observations of the four ground state transitions of the hydroxyl radical (OH) have emerged as an informative tracer of molecular gas in the Galactic interstellar medium. In this talk I'll 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. Of the 30 examples of the flipseen in the literature, 27 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 toward bright background continuum, many (perhaps all) show stimulated emission, and 23 of these are coincident in on-sky position and velocity with HII 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 HII region, which collides with cooler and more diffuse gas hosting the 1612-MHz stimulated emission. We employ non-LTE excitation modelling to test this scenario, and find that indeed FIR emission from warm dust adjacent to the HII 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.