Karl Menten
Max Planck Institute for Radio Astronomy
OH and CH in the Envelopes of Young High Mass Stellar Objects
The OH and CH radicals are ubiquitously found in the interstellar medium. Both probe gas in diffuse and translucent molecular clouds, including the CO-dark component, and also the denser material in star forming regions. Since the beginning of molecular radio astronomy, emission (and absorption) from OH’s radio wavelength hyperfine structure (HFS) lines from a variety of rotationally excited states (with energies up to > 500 K above the ground state) have been found in the closest vicinity of many high mass young stellar objects, prominently, e.g., as strong maser emission in the 18 cm ground state lines. Observations of far infrared (FIR) rotational OH lines with GREAT on SOFIA have confirmed and augmented the observational view of this molecule in compact and ultracompact HII regions. In contrast, radio HFS and submillimeter and FIR rotational lines from rotationally exited CH, observed with Herschel/HIFI and SOFIA/GREAT, have been detected toward only very few such sources and only from the ground and the lowest energy excited level. In combination, both types of lines, modeled using recently calculated new collisional rate coefficients, provide a consistent picture of their emission regions that explains the OH/CH dichotomy.
