Letter to the Editor

Far-infrared detection of OH towards Sagittarius B2 ^*

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: epoleham@mpifr-bonn.mpg.de
² Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, UK e-mail: john.m.brown@chem.ox.ac.uk
³ Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK e-mail: B.M.Swinyard@rl.ac.uk
⁴ Laboratoire d'Astrophysique de Marseille, CNRS & Université de Provence, BP 8, 13376 Marseille Cedex 12, France e-mail: Jean-Paul.Baluteau@oamp.fr

Corresponding author: E. Polehampton, epoleham@mpifr-bonn.mpg.de

Received: 31 January 2003
Accepted: 19 June 2003

Abstract

The frequencies and line strengths of the /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^-1 with the Fabry-Pérot mode of the ISO Long Wavelength Spectrometer. The corresponding transition of ¹⁸OH was also observed and its line shape was modelled using measurements. The ¹⁸O/¹⁷O ratio of 3.5 was then used to compare this with the observed ¹⁷OH line shape.