2019ApJ...881..143H

We report the first detection of C₂ A¹Π_u-X¹Σ_g⁺ (0,0) and CN A²Π_u-X²Σ⁺ (0,0) absorption bands in the interstellar medium. The detection was made using the near-infrared (0.91-1.35 µm) high-resolution (R = 20,000 and 68,000) spectra of Cygnus OB2 No. 12 collected with the WINERED spectrograph mounted on the 1.3 m Araki telescope. The A-X (1,0) bands of C₂ and CN were detected simultaneously. These near-infrared bands have larger oscillator strengths, compared with the A-X (2,0) bands of C₂ and CN in the optical. In the spectrum of the C₂ (0,0) band with R = 68,000, three velocity components in the line of sight could be resolved and the lines were detected up to high rotational levels (J'' ∼ 20). By analyzing the rotational distribution of C₂, we could estimate the kinetic temperature and gas density of the clouds with high accuracy. Furthermore, we marginally detected weak lines of ¹²C¹³C for the first time in the interstellar medium. Assuming that the rotational distribution and the oscillator strengths of the relevant transitions of ¹²C₂ and ¹²C¹³C are the same, the carbon isotope ratio was estimated to be ¹²C/¹³C = 50-100, which is consistent with the ratio in the local interstellar medium. We also constrained the oscillator strength ratio of the C₂ (0,0) and (1,0) bands, for which there exists a discrepancy between theoretical calculations and experimental results. This unique constraint obtained from astronomical observation will contribute to improving the accuracy of the oscillator strength measurement, which will lead to further advancements of the C₂ excitation model and allow the physical conditions of clouds to be derived.