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 ( catalog X Per ) with the Space Telescope Imaging Spectrograph Based on observations obtained with the NASA/ESA Hubble Space Telescope ( HST ) through the Space Telescope Science Institute , which is operated by the Association of Universities for Research in Astronomy , Inc. , under NASA contract NAS5-26555 . .
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 \pm 3600 .
Simultaneously , the second ultraviolet detection of interstellar ^ { 12 } C ^ { 18 } O establishes its isotopomeric ratio at 3000 \pm 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 \pm 12 toward X Per , which is indistinguishable from ^ { 12 } C/ ^ { 13 } C , the result of a balance between photodissociation of ^ { 13 } C ^ { 16 } O and its preferential formation via the isotope exchange reaction between CO and C ^ { + } .