2005A&A...437..957P

As an independent check on previous measurements of the isotopic abundance of oxygen through the Galaxy, we present a detailed analysis of the ground state rotational lines of ¹⁶OH and ¹⁸OH in absorption towards the giant molecular cloud complex, Sagittarius B2. We have modelled the line shapes to separate the contribution of several galactic clouds along the line of sight and calculate ¹⁶OH/¹⁸OH ratios for each of these features. The best fitting values are in the range 320-540, consistent with the previous measurements in the Galactic Disk but slightly higher than the standard ratio in the Galactic Centre. They do not show clear evidence for a gradient in the isotopic ratio with galactocentric distance. The individual ¹⁶OH column densities relative to water give ratios of [H₂O/OH]=0.6-1.2, similar in magnitude to galactic clouds in the sight lines towards W51 and W49. A comparison with CH indicates [OH/CH] ratios higher than has been previously observed in diffuse clouds. We estimate OH abundances of 10^–7-10^–6 in the line of sight features.