Ultraviolet detection of interstellar 12C17O and the CO isotopomeric ratios toward X Persei

We report the detection of fully resolved absorption lines of A-X bands from interstellar (CO)-C-12-O-17 and (CO)-C-12-O-18, through high-resolution spectroscopy of X Persei with the Space Telescope Imaging Spectrograph.(3) The first ultraviolet measurement of an interstellar (CO)-C-12-O-17 column density shows that its isotopomeric ratio is (CO)-C-12-O-16/C-12(17) = 8700+/-3600. Simultaneously, the second ultraviolet detection of interstellar (CO)-C-12-O-18 establishes its isotopomeric ratio at 3000+/-600. These ratios are about five times higher than local ambient oxygen isotopic ratios in the ISM. Such severe fractionation of rare species shows that both (CO)-C-12-O-17 and (CO)-C-12-O-18 are destroyed by photodissociation, whereas (CO)-C-12-O-16 avoids destruction through self-shielding. This is to be contrasted with our ratio of (CO)-C-12-O-16/(CO)-C-13-O-16 = 73+/-12 toward X Per, which is indistinguishable from C-12/C-13, the result of a balance between the photodissociation of (CO)-C-13-O-16 and its preferential formation via the isotope exchange reaction between CO and C+.