First detection of $A$--$X$ (0,0) bands of interstellar C$_2$ and CN
  We report the first detection of C$_2$ $A^1\Pi_u$--$X^1\Sigma_g^+$ (0,0) and
CN $A^2\Pi_u$--$X^2\Sigma^+$ (0,0) absorption bands in the interstellar medium.
The detection was made using the near-infrared (0.91--1.35 $\mu$m)
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 C$_2$ and CN were detected simultaneously. These
near-infrared bands have larger oscillator strengths, compared with the
$A$--$X$ (2,0) bands of C$_2$ and CN in the optical. In the spectrum of the
C$_2$ (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 ($J''\sim20$). By analyzing the rotational distribution of C$_2$, we
could estimate the kinetic temperature and gas density of the clouds with high
accuracy. Furthermore, we marginally detected weak lines of $^{12}$C$^{13}$C
for the first time in the interstellar medium. Assuming that the rotational
distribution and the oscillator strengths of the relevant transitions of
$^{12}$C$_2$ and $^{12}$C$^{13}$C are the same, the carbon isotope ratio was
estimated to be $^{12}\text{C}/^{13}\text{C}=50$--100, which is consistent with
the ratio in the local interstellar medium. We also calculated the oscillator
strength ratio of the C$_2$ (0,0) and (1,0) bands from the observed band
strengths. Unfortunately, our result could not discern theoretical and
experimental results because of the uncertainties. High-resolution data to
resolve the velocity components will be necessary for both bands in order to
put stronger constraints on the oscillator strength ratios.
