2010ApJ...725L.181P

A new interstellar molecule, KCN, has been identified toward the circumstellar envelope of the carbon-rich asymptotic giant branch star, IRC+10216–the fifth metal 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 the most sensitive astronomical spectra at λ ∼ 1 and 3 mm obtained 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~ 1.0x10¹²/cm2 with a rotational temperature of T_rot∼ 53 K. The fractional abundance was calculated to be f(KCN/H₂) ∼ 6x10^–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.