Far-infrared detection of 17OH towards Sagittarius B2

The frequencies and line strengths of the 2Pi3/2 J=5/2-3/2 rotational transition of 17OH 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 17OH 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 with the Fabry-Perot mode of the ISO Long Wavelength Spectrometer. The corresponding transition of 18OH was also observed and its line shape was modelled using HI measurements. The 18O/17O ratio of 3.5 was then used to compare this with the observed 17OH line shape.