Interstellar Detection of Methyl Isocyanate CH$_3$NCO in Sgr B2(N): A Link from Molecular Clouds to Comets

Abstract

A new interstellar molecule, CH$_3$NCO (methyl isocyanate), has been detected using the 12 m telescope of the Arizona Radio Observatory (ARO). CH$_3$NCO 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 $\rightarrow$ 7, 9 $\rightarrow$ 8, 10 $\rightarrow$ 9, 11 $\rightarrow$ 10, and 12 $\rightarrow$ 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 seventeen lines appearing as distinct, uncontaminated spectral features, clearly showing the classic a-type, asymmetric top pattern, with T$_R$* ~ 20-70 mK. The CH$_3$NCO spectra also appear to exhibit two velocities components near V$_{LSR}$ ~ 62 and 73 km s$^{-1}$, both with $Δ$V$_{1/2}$ ~ 10 km s$^{-1}$ - typical of molecules such as CH$_2$CHCN, HNCO, and HCOOCH$_3$ in Sgr B2(N). The column density of CH$_3$NCO in Sgr B2(N) was determined to be N$_{tot}$ ~ 2.3 x 10$^{13}$ cm$^{-2}$ 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 ~ 7.6 x 10$^{-12}$ and 5.0 x 10$^{-12}$, respectively. CH$_3$NCO 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), CH$_3$NCO is roughly ~0.04% of the H$_2$O abundance.