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[71.09] Further Evidence for Chemical Fractionation from Ultraviolet Observations of Interstellar Carbon Monoxide

S.R. Federman (UToledo), David L. Lambert, Yaron Sheffer (UTexas), Jason A. Cardelli (Villanova), B-G Andersson (JHU/JPL), Ewine F. van Dishoeck (Leiden), J. Zsarg\'{o} (UToledo)

Ultraviolet absorption from interstellar ¹²CO and ¹³CO was detected toward \rho Oph A and \chi Oph. The measurements were obtained at medium resolution with the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Equivalent widths were converted into column densities through the use of f-values from Chan et al. (1993, Chem. Phys., 170, 123). Column density ratios, N(¹²CO)/N(¹³CO), of 125 ± 23 and 117 ± 35 were derived for the sight lines toward \rho Oph A and \chi Oph, respectively. A value of 1000 ± 500 for the ratio N(¹²C¹⁶O)/N(¹²C¹⁸O) toward \rho Oph A was also obtained.

These ratios are larger than the isotopic ratios for carbon and oxygen appropriate for the ambient material. Since for both carbon and oxygen the more abundant isotopic variant is enhanced, selective isotope photodissociation plays the key role in the fractionation process. The enhancement arises because the more abundant variant has lines that are more optically thick, resulting in more self shielding from dissociating radiation. A simple argument involving the amount of self shielding (from N(¹²CO)) and the strength of the UV field permeating the gas (from the amount of vibrationally excited H₂ seen in our spectra) shows that selective isotope photodissociation does control the fractionation seen in these two sight lines, as well as the sight line to \zeta Oph.

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