17O and 18O Abundances in Interstellar Clouds
The determination of isotopic abundance ratios in interstellar molecules is a fast-growing subject (see the review by Wannier in this book). Because CO is so widespread in our Galaxy, its isotopic substitutions are of particular importance. We concentrate here on the two isotopic molecules 12C17O and 12C18O. 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 13C16O 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 (Watson, this book), is unknown; thus we do not know to which extent the 17O/18O ratio is equal to the C17O/C18O ratio.
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