Burst star formation and the initial mass function in W51 region

Abstract

Near-infrared observations of the central part of W51 (G49.5 - 0.4) have been carried out. Imaging data were obtained over an area of 15 squared arcmin, centered near W51 IRS (Infrared Source) 2. K-band spectroscopic data were also obtained for selected objects in this area. More than 7,000 point sources were detected from the imaging data, and about 170 sources were identified as physically associated with G49.5-0.4 star forming region. Ages of the HII regions were inferred from the sizes of nebulosities on the Br(gamma) image. Coupling the information of the age of HII region and the luminosity of exiting star, the mass and age for each source were estimated, and Initial Mass Function (IMF) of massive stars in G49.5-0.4 was derived. In the mass range between 10 solar mass and 30 solar mass, the IMF slope is consistent with the value -1.8, which is derived for field stars. However, there is an excess of stellar population in the range above 30 solar mass. Four subgroups were also defined in this region. In the youngest group, 17 O stars were born during the last 0.8 Myr. The massive star forming activity of this group is more violent than any other visible Galactic HII regions.

W51(G49.5-0.4)の中心部の近赤外線観測を行った。W51内のIRS2(赤外線源2)を中心とした15分角の領域全体の撮像データを得た。また、この領域内で選んだ天体について、Kバンドの分光データも得た。撮像から7,000個以上の赤外線源が検出され、そのうち170個が物理的にG49.5-0.4に付随する赤外線源であることが特定された。HII領域の年齢は、Brγ線の撮像で得られた星雲の大きさから求められた。HII領域の年齢と励起星の光度の情報を組み合わせることにより、各赤外線源の質量と年齢を推定し、G49.5-0.4内の大質量星の初期質量関数が求まった。領域内の星について、10太陽質量から30太陽質量の範囲では、IMF(初期質量関数)の傾きは乗数-1.8で一定であった。しかし、30太陽質量以上の範囲では、星の数に超過が見られた。また4つのサブグループに分けた。最も若いグループでは、過去80万年の間に、17個のO型星が生まれた。このグループの大質量星生成活動は銀河系内のどの可視光で見えるHII領域よりも活動が激しい。