INTERSTELLAR DETECTION OF METHYL ISOCYANATE CH3NCO IN Sgr B2(N): A LINK FROM MOLECULAR CLOUDS TO COMETS

A new interstellar molecule, CH3NCO (methyl isocyanate), has been detected using the 12 m telescope of the Arizona Radio Observatory (ARO). CH3NCO was identified in spectra covering 48 GHz (68-116 GHz) in the 3 mm segment of a broadband survey of Sgr B2(N). Thirty very favorable rotational lines (K-a = 0 and K-a = 1 only; E-u < 60 K) originating in five consecutive transitions (J - 8 -> 7, 9 -> 8, 10 -> 9, 11 -> 10, and 12 -> 11) in both the A and E internal rotation species are present in this frequency range. Emission was observed at all of the predicted frequencies, with 17 lines appearing as distinct, uncontaminated spectral features, clearly showing the classic a-type, asymmetric top pattern, with T-R* approximate to 20-70 mK. The CH3NCO spectra also appear to exhibit two velocity components near V-LSR approximate to 62 and 73 km s(-1), both with Delta V-1/2 approximate to 10 km s(-1)-typical of molecules such as CH2CHCN, HNCO, and HCOOCH3 in Sgr B2(N). The column density of CH3NCO in Sgr B2(N) was determined to be N-tot approximate to 2.3 x 10(13) and 1.5 x 10(13) cm(-2) for the 62 and 73 km s(-1) components, corresponding to fractional abundances, relative to H-2, of f approximate to 7.6 x 10(-12) and 5.0 x 10(-12), respectively. CH3NCO was recently detected in volatized material from comet 67P/Churyumov-Gerasimenko by Rosetta's Philae lander, with an abundance similar to 1.3% of water; in Sgr B2(N), CH3NCO is roughly similar to 0.04% of the H2O abundance.