CARBON ISOTOPE RATIO IN ~(12)CO/~(13)CO TOWARD LOCAL MOLECULAR CLOUDS WITH NEAR-INFRARED HIGH-RESOLUTION SPECTROSCOPY OF VIBRATIONAL TRANSITION BANDS

摘要

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 ~(12)CO ν = 2←0 and ~(13)CO ν = 1 ← 0 were observed with high-resolution near-infrared spectroscopy (R = 23,000) to measure the ~(12)CO/~(13)CO ratio. The isotopic ratios are ~(12)CO/~(13)CO = 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 ~(12)CO/~(13)CO = 57 +- 5 from millimeter C~(18)O emission observations. This is not easily explained by saturation of the ~(13)CO absorption. It is also questionable whether the selective photodestruction of ~(13)CO can account for the difference between the Galactic trend and the present observation, because the molecular clouds are with high visible extinction (A_ν = 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 ~(12)CO is twice that of ~(13)CO. We attribute the temperature discrepancy to the photon-trapping effect, which makes the radiative cooling of the main isotopomer less effective.