IDENTIFICATION OF KCN IN IRC+10216: EVIDENCE FOR SELECTIVE CYANIDE CHEMISTRY

Abstract

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{sub a} = 1 and 2 asymmetry components of the J = 11 {yields} 10 and J = 10 {yields} 9 transitions, i.e., the K-ladder structure distinct to an asymmetric top. These data are some of the most sensitive astronomical spectra at {lambda} {approx} 1 and 3 mm obtained to date, with 3{sigma} noise levels {approx}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{sub outer} {approx} 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{sub tot} {approx} 1.0 x 10{sup 12} cm{sup -2} with a rotational temperature ofmore » T{sub rot} {approx} 53 K. The fractional abundance was calculated to be f(KCN/H{sub 2}) {approx} 6 x 10{sup -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.« less