First detection of A–X (0,0) bands of interstellar C2 and CN.

We report the first detection of C2 P - S +A Xu g 11 (0,0) and CN P - S +A Xu 22 (0,0) absorption bands in the interstellar medium. The detection was made using the near-infrared (0.91–1.35μm) high-resolution (R=20,000 and 68,000) spectra of Cygnus OB2 No. 12 collected with the WINERED spectrograph mounted on the 1.3m 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.