2008ApJ...688L..83C

We report the detection in the envelope of the C-rich star IRC +10216 of four series of lines with harmonically related frequencies: B1389, B1390, B1394, and B1401. The four series must arise from linear molecules with mass and size close to those of C₆ H and C₅N. Three of the series have half-integer rotational quantum numbers; we assign them to the ²Δ and ²Σ^– vibronic states of C₆ H in its lowest (ν₁₁) bending mode. The fourth series, B1389, has integer J with no evidence of fine or hyperfine structure; it has a rotational constant of 1388.860(2) MHz and a centrifugal distortion constant of 33(1) Hz; it is almost certainly the C₅N^– anion.