Ultraviolet Detection of Interstellar [TSUP]12[/TSUP]C[TSUP]17[/TSUP]O and the CO Isotopomeric Ratios toward X Persei

We report the detection of fully resolved absorption lines of A-X bands from interstellar 12 C 17 O and 12 C 18 O, through high-resolution spectroscopy of X Persei with the Space Telescope Imaging SpectrographBased on observations obtained with the NASA/ESA Hubble Space Telescope through the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.. The first ultraviolet measurement of an interstellar 12 C 17 O column density shows that its isotopomeric ratio is 12 C 16 O/ 12 C 17 O = 8700 ± 3600. Simultaneously, the second ultraviolet detection of interstellar 12 C 18 O 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 12 C 17 O and 12 C 18 O are destroyed by photodissociation, whereas 12 C 16 O avoids destruction through self-shielding. This is to be contrasted with our ratio of 12 C 16 O/ 13 C 16 O = 73 ± 12 toward X Per, which is indistinguishable from 12 C/ 13 C, the result of a balance between the photodissociation of 13 C 16 O and its preferential formation via the isotope exchange reaction between CO and C + .