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

We report the detection of fully resolved absorption lines of A−X bands from interstellar CO and CO, through high-resolution spectroscopy of X Per with the Space Telescope Imaging Spectrograph. The first ultraviolet measurement of an interstellar CO column density shows that its isotopomeric ratio is CO/CO = 8700 ± 3600. Simultaneously, the second ultraviolet detection of interstellar CO 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 and CO are destroyed by photodissociation, whereas CO avoids destruction through self-shielding. This is to be contrasted with our ratio of CO/CO = 73 ± 12 toward X Per, which is indistinguishable from C/C, the result of a balance between photodissociation of CO and its preferential formation via the isotope exchange reaction between CO and C. Subject headings: ISM: abundances — ISM: molecules — molecular data — stars: individual (X Per) — ultraviolet: ISM