NGC 4038/39中一个年轻超星团的恒星形成历史：低质量前主序星的直接探测
Star Formation History of a Young Super-Star Cluster in NGC 4038/39: Direct Detection of Low Mass Pre-Main Sequence Stars
摘要：
本文利用星族合成模型分析了位于相互作用星系NGC 4038/39重叠区的一个年轻大质量星团的近红外光谱。我们的目标是对集群人口进行建模，并对其初始质量函数（IMF）提供粗略的约束。该星团显示出年轻的迹象，如热无线电发射和近红外的强氢发射线。晚型吸收线也是星团中晚型恒星的标志。这些吸收线的强度和比例不能通过晚型前主序星（PMS）或红超巨星单独复制。因此，我们将光谱解释为两个不同年龄的星团的叠加，这是可行的，因为1”光谱涵盖了~90 pc的物理区域，并且超级星团的半径通常被测量为几个秒差距。一个星团是年轻的（<= 300万年），负责部分晚型吸收特征，这是由于星团中的PMS星和氢发射线。第二个星团较老（600万年-1800万年），需要重现光谱中晚期吸收特征的整体深度。两者都需要精确地再现物体的近红外光谱。因此，我们已经直接检测PMS对象在一个未解决的超级星团的第一次使用人口合成模型和主序前轨道相结合。这项分析作为我们的技术，以限制年轻的超级星团的低质量IMF以及探索年轻的UC HII地区的恒星形成历史的测试平台。
英文摘要：
We present an analysis of the near-infrared spectrum of a young massive star cluster in the overlap region of the interacting galaxies NGC 4038/39 using population synthesis models. Our goal is to model the cluster population as well as provide rough constraints on its initial mass function (IMF). The cluster shows signs of youth such as thermal radio emission and strong hydrogen emission lines in the near-infrared. Late-type absorption lines are also present which are indicative of late-type stars in the cluster. The strength and ratio of these absorption lines cannot be reproduced through either late-type pre-main sequence (PMS) stars or red supergiants alone. Thus we interpret the spectrum as a superposition of two star clusters of different ages, which is feasible since the 1" spectrum encompasses a physical region of ~90 pc and radii of super-star clusters are generally measured to be a few parsecs. One cluster is young (<= 3 Myr) and is responsible for part of the late-type absorption features, which are due to PMS stars in the cluster, and the hydrogen emission lines. The second cluster is older (6 Myr - 18 Myr) and is needed to reproduce the overall depth of the late-type absorption features in the spectrum. Both are required to accurately reproduce the near-infrared spectrum of the object. Thus we have directly detected PMS objects in an unresolved super-star cluster for the first time using a combination of population synthesis models and pre-main sequence tracks. This analysis serves as a testbed of our technique to constrain the low-mass IMF in young super-star clusters as well as an exploration of the star formation history of young UC HII regions.
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