Identification of KCN in IRC+10216: Evidence for selective cyanide chemistry

A new interstellar molecule, KCN, has been identified toward the circumstellar envelope of the carbon-rich asymptotic giant branch star, IRC+10216 - the fifthmetal cyanide species to be detected in space. Fourteen rotational transitions of this T-shaped, asymmetric top were searched for in the frequency range of 83-250 GHz using the Arizona Radio Observatory (ARO) 12 m Kitt Peak antenna, the IRAM 30 m telescope, and the ARO Submillimeter Telescope. Distinct lines were measured for 10 of these transitions, including the K _a = 1 and 2 asymmetry components of the J = 11→10 and J = 10→9 transitions, i.e., the K-ladder structure distinct to an asymmetric top. These data are some of themost sensitive astronomical spectra at λ∼1 and 3mmobtained to date, with 3σ noise levels ∼0.3 mK, made possible by new ALMA technology. The line profiles from the ARO and IRAM telescopes are consistent with a shell-like distribution for KCN with r _outer ∼ 15″, but with an inner shell radius that extends into warmer gas. The column density for KCN in IRC+10216 was found to be N _tot ≈ 1.0×10¹² cm^-2 with a rotational temperature of T_rot ∼ 53 K. The fractional abundance was calculated to be f(KCN/H₂) ∼ 6×10^-10, comparable to that of KCl. The presence of KCN in IRC+10216, along with MgNC, MgCN, NaCN, and AlNC, suggests that cyanide/isocyanide species are the most common metal-containing molecules in carbon-rich circumstellar gas.