DETECTION OF CH3NCO IN THE GALACTIC CENTER STAR-FORMING REGION SAGITTARIUS B2(M) BY RADIO ASTRONOMICAL OBSERVATIONS

Large difference of chemical compositions between molecular clouds and comets is a big question for astrochemistry. The case of a pre-biotic molecule \chem{CH_3NCO} is one of them. The abundance ratio of [\chem{CH_3NCO}]/[\chem{HNCO}] is high in the comet 67P ($>$ 4, [1]), although it is low (0.02 $-$ 0.3, e.g., [2]) in molecular clouds. An abundance of \chem{CH_3NCO} is expected to be held and/or increased during evolutionary process of a cloud. A pair of an old core and a young core having the similar chemical compositions needs to be investigated for this evolutionary process. In this work, we aimed to detect \chem{CH_3NCO} in the middle (M) core, which is relatively older than the north (N) core, in the Galactic Center star-forming region Sagittarius B2 with the 45 m telescope of Nobeyama Radio Observatory. The rotational transitions of $J$ = 10 $\rightarrow$ 9 to 13 $\rightarrow$ 12 for \chem{CH_3NCO} were detected in the 85 $-$ 114 GHz region. The column density and the rotational temperature are derived to be $N$ = (4.3 $\pm$ 2.1) $\times$ $10^{13}$ cm$^{-2}$ and $T_{rot}$ = (32 $\pm$ 9) K, respectively, assuming local thermal equilibrium. Similarly, an abundance of \chem{HNCO} is estimated to be $N$ = (1.3 $\pm$ 0.5) $\times$ $10^{15}$ cm$^{-2}$ ($T_{rot}$ = 21 $\pm$ 2 K), giving the ratio of [\chem{CH_3NCO}]/[\chem{HNCO}] = 0.032. Thus, as a simplest model, it is suggested that an abundance of \chem{CH_3NCO} is held during evolutionary process of the Sagittarius B2 region. \\ \noindent [1] Goesmann et al., Science, 349, 689 (2015). [2] Halfen et al., ApJ, 812, L5 (2015)