2015ApJ...813..120S

This study reports an unusual heterogeneity in [¹²C¹⁶O]/[¹³C¹⁶O] abundance ratios of carbon monoxide observed in the gas phase toward seven ∼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 µm fundamental and 2.3 µm overtone rovibrational bands of CO at very high spectral resolution (λ/Δλ ≈ 95,000), observed with the Cryogenic Infrared Echelle Spectrograph (CRIRES) on the Very Large Telescope. We find [¹²C¹⁶O]/[¹³C¹⁶O] values ranging from ∼85 to 165, significantly higher than those of the local interstellar medium (ISM) (∼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 ¹²C/¹³C 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 [¹²C¹⁶O]/[¹³C¹⁶O] ratios observed in this study. Comparison of the solid CO against gas-phase [¹²C¹⁶O]/[¹³C¹⁶O] suggests that interactions between CO ice and gas reservoirs need to be further investigated as at least a partial explanation for the unusually high [¹²C¹⁶O]/[¹³C¹⁶O] observed.