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

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  • Mitsunori Araki
    Department of Chemistry, Faculty of Science Division I, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
  • Shuro Takano
    Department of Physics, General Studies, College of Engineering, Nihon University, 1 Nakagawara, Tokusada, Tamuramachi, Koriyama, Fukushima 963-8642, Japan
  • Nobuhiko Kuze
    Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan
  • Yoshiaki Minami
    Department of Chemistry, Faculty of Science Division I, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
  • Takahiro Oyama
    Department of Chemistry, Faculty of Science Division I, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
  • Kazuhisa Kamegai
    Astronomy Data Centre, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
  • Yoshihiro Sumiyoshi
    Division of Pure and Applied Science, Graduate School of Science and Technology, Gunma University, 4-2 Aramaki, Maebashi, Gunma 371-8510, Japan
  • Koichi Tsukiyama
    Department of Chemistry, Faculty of Science Division I, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan

抄録

<jats:title>ABSTRACT</jats:title> <jats:p>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).</jats:p>

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