INTERSTELLAR CARBON-MONOXIDE TOWARD ZETA-OPHIUCHI

Interstellar CO A-X bands in the spectrum of zeta Oph were recorded at high S/N with grating G160M of the Goddard High Resolution Spectrograph on the Hubble Space Telescope, Isotopic fractionation of CO is severe: (CO)-C-12/(CO)-C-13 = 167, (CO)-O-16/(CO)-O-18 similar or equal to 1550 and (CO)-O-16/(CO)-O-17 > 5900 are found where C-12/C-13 = 70, O-16/ O-18 = 500, and O-16/O-17 = 2600 are observed or expected. Standard models of the zeta Oph cloud predict (CO)-C-12/(CO)-C-13 less than or similar to 70. The higher observed ratio suggests that photodissociation of CO, not the isotopic charge exchange reaction (C-13(+) + CO <----> C-12(+) + (CO)-C-13), is the dominant influence on the (CO)-C-12/(CO)-C-13 ratio.