The Carbon Isotope Ratio in Local Molecular Clouds

Abstract

We report the carbon isotope ratio in nearby molecular clouds 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 relative abundance of ¹³CO to ¹²CO. The isotopic ratios are ¹²CO/¹³CO = 137±9 (LkHα 101), 86±49 (AFGL 490), and 158 (Mon R2 IRS 3), which is twice higher than in the solar neighborhood. The molecular clouds are with high visible extinction (A _V = 10–70 mag), well shielded from destructive FUV field. It is questionable that the selective photo-destruction of ¹³CO plays a major role in biasing isotope ratio. Uncertainty in the Doppler parameters of the unresolved absorption lines, and possible emission filling of fundamental transitions are suspected to account for the high ¹²CO/¹³CO ratio. Higher resolution spectroscopy (R ~ 100,000) is the key to go for the accurate measurement of isotope ratio.