High-precision Co, Co and Comeasurements in Young Stellar Objects: Analogues for Co Self-shielding in the Early Solar System

CO SELF-SHIELDING IN THE EARLY SOLAR SYSTEM. Rachel L. Smith, Klaus M. Pontoppidan, Edward D. Young, Mark R. Morris, and Ewine F. van Dishoeck, Department of Earth and Space Sciences, University of California Los Angeles (UCLA) ([email protected]), Hubble Fellow, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA ([email protected]), Institute of Geophysics and Planetary Physics, UCLA ([email protected]), Division of Astronomy and Astrophysics, Department of Physics and Astronomy, UCLA ([email protected]), Leiden Observatory, Leiden University, P.O. Box 9513, NL 2300 RA Leiden, The Netherlands ([email protected]), Max Planck Institut für Extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany Introduction: The oxygen isotope anomaly in the Solar System, defined as a mass-independent correlation between [O]/[O] and [O]/[O] among rocky bodies, has defied explanation since its discovery [1]. One leading explanation is CO self-shielding [2], whereby the isotopologues of CO are shielded from further photodissociation by far ultraviolet (FUV) radiation in proportion to their abundances. Thus, CO and CO are more rapidly destroyed than CO. While isotope-selective photodissociation is known to occur in molecular clouds [3,4,5], and possibly in the HL Tau disk [6], conclusive evidence in circumstellar disks is lacking. Until recently, searching for oxygen isotope fractionation in protoplanetary disks has been difficult due to the minimum required ∼10% accuracy in CO isotopologue measurements. Recently, the advent of improved astronomical instrumentation has permitted precise comparisons between isotope ratios in protoplanetary systems and meteorites. Here we report new, very high-resolution infrared observations of four CO isotopologues – CO, CO, CO and the rare species, CO – in two YSOs, and evidence for CO selfshielding in a circumstellar disk.