Carbon Isotope Ratio in ¹²CO/¹³CO toward Local Molecular Clouds with Near-Infrared High-Resolution Spectroscopy of Vibrational Transition Bands^*

We report the carbon monoxide isotope ratio in local molecular clouds toward LkHα 101, AFGL 490, and Mon R2 IRS 3. The vibrational transition bands of ¹²CO ν = 2 ← 0 and ¹³CO ν = 1 ← 0 were observed with high-resolution near-infrared spectroscopy (R = 23,000) to measure the ¹²CO/¹³CO ratio. The isotopic ratios are ¹²CO/¹³CO = 137 ± 9 (LkHα 101), 86 ± 49 (AFGL 490), and 158 (Mon R2 IRS 3), which are 1.5-2.8 times higher than the local interstellar medium value of ¹²CO/¹³CO = 57 ± 5 from millimeter C¹⁸O emission observations. This is not easily explained by saturation of the ¹³CO absorption. It is also questionable whether the selective photodestruction of ¹³CO can account for the difference between the Galactic trend and the present observation, because the molecular clouds are with high visible extinction (A_V = 10-70 mag), well shielded from destructive FUV radiation. The molecular gas associated with AFGL 490 and Mon R2 IRS 3 consists of multiple temperature components lying in the lines of sight. In the cool component (T_ex < 100 K), the excitation temperature of ¹²CO is twice that of ¹³CO. We attribute the temperature discrepancy to the photon-trapping effect, which makes the radiative cooling of the main isotopomer less effective.