Further evidence for chemical fractionation from ultraviolet observations of carbon monoxide

Ultraviolet absorption from interstellar ¹²CO and ¹³CO was detected toward p Oph A and x 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 p Oph A and x Oph, respectively. A value of 1100 ± 600 for the ratio N(¹²C¹⁶O)/ N(¹²C ¹⁸O) toward p Oph A was also obtained. Absorption from vibrationally excited H₂ (v″ = 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 ζ Oph.