Heterogeneity in $^{12}$CO/$^{13}$CO Ratios Toward Solar-Type Young
  Stellar Objects
  This study reports an unusual heterogeneity in
[$^{12}$C$^{16}$O]/[$^{13}$C$^{16}$O] abundance ratios of carbon monoxide
observed in the gas phase toward seven ~ solar-mass YSOs and three dense
foreground clouds in the nearby star-forming regions, Ophiuchus, Corona
Australis, Orion, 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 resolution
($\lambda$/$\Delta$$\lambda\approx 95,000$), observed with the CRIRES
spectrograph on the Very Large Telescope. We find
[$^{12}$C$^{16}$O]/[$^{13}$C$^{16}$O] values ranging from ~ 85 to 165,
significantly higher than those of the local interstellar medium (~ 65 to 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 $^{12}$C/$^{13}$C 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 [$^{12}$C$^{16}$O]/[$^{13}$C$^{16}$O] ratios
observed in this study. Comparison of the solid CO against gas-phase
[$^{12}$C$^{16}$O]/[$^{13}$C$^{16}$O] suggests that interactions between CO ice
and gas reservoirs need to be further investigated as at least a partial
explanation for the unusually high [$^{12}$C$^{16}$O]/[$^{13}$C$^{16}$O]
observed.
