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 ≈ 20-70 mK. The CH₃NCO spectra also appear to exhibit two velocity components near V_LSR ≈ 62 and 73 km s^-1, both with ΔV_1/2 ≈ 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 ≈ 2.3 � 10¹³ and 1.5 � 10¹³ cm^-2 for the 62 and 73 km s^-1 components, corresponding to fractional abundances, relative to H₂, of f ≈ 7.6 � 10^-12 and 5.0 � 10^-12, respectively. CH₃NCO was recently detected in volatized material from comet 67P/Churyumov-Gerasimenko by Rosetta's Philae lander, with an abundance ∼1.3% of water; in Sgr B2(N), CH₃NCO is roughly ∼0.04% of the H₂O abundance.