17O and 18O abundances in interstellar clouds

The determination of isotopic abundance ratios in interstellar molecules is a fast-growing subject. Because CO is so widespread in our Galaxy, its isotopic substitutions are of particular importance. In this paper the two isotopic molecules 12C17O and 12C18O are considered. They give only faint J = 1 - 0 lines, at 112.36 GHz and 109.78 GHz respectively, but these lines are unsaturated even in Sgr B2 (as can be checked by comparing them to the lines of the more abundant species 12C16O and 13O16O in the same molecular clouds), and presumably their excitation conditions are similar; thus the ratio of their intensities directly provides an abundance ratio. Whether chemical fractionation of the oxygen isotopes occurs, as it might be the case for the carbon isotopes, is unknown; thus it is not known to what extent the 17O/18O ratio is equal to the C17O/C18O ratio. (Auth.)