Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide

Ultraviolet absorption from interstellar 12CO and 13CO was detected toward Oph A and Oph. The measurements were obtained at medium resolution with the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Column density ratios, N(12CO)/N(13CO), of 125 23 and 117 35 were derived for the sight lines toward Oph A and Oph, respectively. A value of 1100 600 for the ratio N(12C16O)/ N(12C18O) toward Oph A was also obtained. Absorption from vibrationally excited H2 (v00 1⁄4 3) was clearly seen toward this star as well. The ratios are larger than the isotopic ratios for carbon and oxygen appropriate for ambient interstellar material. Since for both carbon and oxygen the more abundant isotopomer is enhanced, selective isotopic photodissociation plays the key role in the fractionation process for these directions. The enhancement arises because the more abundant isotopomer 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 ultraviolet radiation field permeating the gas (from the amount of vibrationally excited H2) shows that selective isotopic photodissociation controls the fractionation seen in these two sight lines, as well as the sight line to Oph. Subject headings: astrochemistry — ISM: abundances — ISM: molecules — stars: individual ( Ophiuchi A, Ophiuchi)