言語種別 英語 発行・発表の年月 2023/02 形態種別 学術雑誌 査読 査読あり 標題 An expanding ring of the hypercompact H ii region W 49 N:A2   An expanding ring of the hypercompact H ii region W 49 N:A2 執筆形態 共著 掲載誌名 Publications of the Astronomical Society of Japan 掲載区分 国外 出版社・発行元 Oxford University Press 巻・号・頁 75(1),pp.225-232 総ページ数 13 担当範囲 執筆，データ解析 担当区分 筆頭著者,責任著者 著者・共著者 ◎Ryosuke Miyawaki, Masahiko Hayashi, Tetsuo Hasegawa   ◎Ryosuke Miyawaki, Masahiko Hayashi, Tetsuo Hasegawa 概要 We present 250 GHz continuum and H29α line data toward W 49 N:A2, a hypercompact H ii region ionized by an O9 star. The data obtained with ALMA at a resolution of ∼0".05 (600 au) confirmed the presence of an ionized ring with a radius of ∼700 au. It has a width of ∼1000 au and is relatively flat with a scale height of less than several hundred au with a velocity of 13.2 km/s. The ring also shows a hint of rotation at 2.7 km/s, which is significantly (2.5 σ) smaller than the Kepler velocity of 5.2 km/s at its radius around the 20 M⊙ star. This can be interpreted as that the ring gas has been transported from the radius of ∼170 au by conserving its original specific angular momentum that it had there. The ionized ring may thus be a remnant of the accretion disk that fed the O9 star. The data has revealed a rare example of how a massive star terminates its accretion at the end of its formation, transforming a hypercompact H ii region into an ultracompact H ii region.