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, a 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 Center region and other sources, using the
Nobeyama 45 m telescope. Based on the detected absorption lines 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) x 10^14 cm-2 were derived for this
molecule, revealing extremely concentrated J = K levels due to the lower
excitation temperature and the 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).
