Observations and analysis of absorption lines including J = K rotational levels of CH3CN: The envelope of Sagittarius B2(M)

Molecules in diffuse and translucent clouds experience cooling as a result of radiation and less excitation from collisions. However, rotation around a molecular axis of acetonitrile, CH3CN, cannot be cooled by radiation, causing rotational populations to concentrate at the J = K levels. We aim to search for absorption lines of CH3CN having J = K level concentrations in diffuse and translucent clouds. The JK = 43-33 transition at 73.6 GHz was investigated toward Sgr B2(M) in the Galactic Centre region and other sources, using the Nobeyama 45-m telescope. Based on the absorption lines detected toward Sgr B2(M), a radiation temperature of 2.8 ± 0.5 K, kinetic temperature of 88 ± 29 K and column density of (1.35 ± 0.14) × 1014 cm -2 were derived for this molecule, revealing extremely concentrated J = K levels due to the lower excitation temperature and higher kinetic temperature. The absorption lines occurred at a velocity of 64 km s-1. The results confirm that CH3CN with J = K level concentrations exists in the envelope of Sgr B2(M).