ar X iv : a st ro - p h / 03 04 17 0 v 1 9 A pr 2 00 3 Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide

Ultraviolet absorption from interstellar CO and CO was detected toward ρ Oph A and χ Oph. The measurements were obtained at medium resolution with the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Column density ratios, N(CO)/N(CO), of 125 ± 23 and 117 ± 35 were derived for the sight lines toward ρ Oph A and χ Oph, respectively. A value of 1100 ± 600 for the ratio N(CO)/N(CO) toward ρ 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(CO)] and the strength of the ultraviolet radiation field premeating 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. Based on observations obtained with the NASA/ESA Hubble Space Telescope through the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606. Guest Observer, McDonald Observatory, University of Texas at Austin. Department of Astronomy, University of Texas, Austin, TX 78712. Department of Astronomy and Astrophysics, Villanova University, Villanova, PA 19085. Deceased. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109. Sterrewacht Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218.