Detection by ISO of the far-infrared OH maser pumping lines in IRC+10420

The population inversions which lead to circumstellar hydroxyl (OH) maser emission from cool stars have long been thought to be radiatively pumped. However, the proposed pumping mechanism has not been directly observed before now, because the spectral lines that correspond to the rotational transitions of the pumping cycle lie in the far-infrared, and are unobservable from the ground. We present ISO observations of IRC+10420, obtained with its Short and Long Wavelength Spectrometers (SWS and LWS, respectively). This object is an F-type hypergiant and a strong OH maser source. The SWS spectrum clearly shows the 34.6-μm doublet in absorption, and the ensuing rotational cascade lines at 98.7,163 and 79 μm are clearly detected in emission in the LWS spectrum. These spectra provide the first direct confirmation of the radiative pumping cycle for circumstellar OH masers.