HETEROGENEITY IN 12CO/13CO ABUNDANCE RATIOS TOWARD SOLAR-TYPE YOUNG STELLAR OBJECTS

This study reports an unusual heterogeneity in [(CO)-C-12-O-16]/[(CO)-C-13-O-16] abundance ratios of carbon monoxide observed in the gas phase toward seven similar to solar-mass young stellar objects (YSOs) and three dense foreground clouds in the nearby star-forming regions, Ophiuchus, Corona Australis, Orion, and Vela, and an isolated core, L43. Robust isotope ratios were derived using infrared absorption spectroscopy of the 4.7 mu m fundamental and 2.3 mu m overtone rovibrational bands of CO at very high spectral resolution (lambda/Delta lambda approximate to 95,000), observed with the Cryogenic Infrared Echelle Spectrograph (CRIRES) on the Very Large Telescope. We find [(CO)-C-12-O-16]/[(CO)-C-13-O-16] values ranging from similar to 85 to 165, significantly higher than those of the local interstellar medium (ISM) (similar to 65-69). These observations are evidence for isotopic heterogeneity in carbon reservoirs in solar-type YSO environments, and encourage the need for refined galactic chemical evolution models to explain the C-12/C-13 discrepancy between the solar system and local ISM. The oxygen isotope ratios are consistent with isotopologue-specific photodissociation by CO self-shielding toward the disks, VV CrA N and HL Tau, further substantiating models predicting CO self-shielding on disk surfaces. However, we find that CO self-shielding is an unlikely general explanation for the high [(CO)-C-12-O-16]/ [(CO)-C-13-O-16] ratios observed in this study. Comparison of the solid CO against gas-phase [(CO)-C-12-O-16]/[(CO)-C-13-O-16] suggests that interactions between CO ice and gas reservoirs need to be further investigated as at least a partial explanation for the unusually high [(CO)-C-12-O-16]/[(CO)-C-13-O-16] observed.