Far - infrared detection of 17 OH towards Sagittarius B 2 ⋆

The frequencies and line strengths of the 2 Π 3/2 J=5/2–3/2 rotational transition of 17 OH have been calculated from an analysis of its far-infrared laser magnetic resonance spectrum. These results have been used to make the first detection of a pure rotational transition of 17 OH in the ISM. Two resolved components of this transition appear in absorption towards the giant molecular cloud Sagittarius B2, which was observed at a spectral resolution of 33 km s −1 with the Fabry-Pérot mode of the ISO Long Wavelength Spectrometer. The corresponding transition of 18 OH was also observed and its line shape was modelled using H  measurements. The 18 O/ 17 O ratio of 3.5 was then used to compare this with the observed 17 OH line shape.