Oxygen isotopic ratios in galactic clouds along the line of sight towards Sagittarius B 2 ⋆

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 16 OH and 18 OH 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 16 OH/ 18 OH 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 16 OH column densities relative to water give ratios of [H 2 O/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.