大质量恒星形成的最小柱密度为1 g cm^-2
A Minimum Column Density of 1 g cm^-2 for Massive Star Formation
摘要：
大质量恒星非常罕见，但它们的极端光度使它们成为我们在遥远星系中观察到的唯一类型的年轻恒星，也是当今宇宙中的主要能量来源。它们很少形成，因为有效的辐射冷却使大多数恒星形成的气体云在坍缩时接近等温，这有利于分裂成小于1 Msun的恒星。通过吸积低质量恒星来加热星云可以防止碎片化，并允许形成大质量恒星，但星云必须具有什么性质才能形成大质量恒星，因此大质量恒星在星系中的形成位置尚未确定。在这里，我们证明了只有柱密度>~ 1 g cm^-2的云才能避免碎裂，形成大质量恒星。这个阈值，以及它所暗示的恒星初始质量函数（IMF）的环境变化，自然地解释了大质量星团的特征柱密度以及星系盘中Halpha和UV辐射的径向分布之间的差异。阈值的存在也意味着，应该有可检测的变化，在IMF与环境内的银河系和星系之间的大质量星团的特征柱密度，并在某些星系环境中的恒星形成率可能被系统地低估。
英文摘要：
Massive stars are very rare, but their extreme luminosities make them both the only type of young star we can observe in distant galaxies and the dominant energy sources in the universe today. They form rarely because efficient radiative cooling keeps most star-forming gas clouds close to isothermal as they collapse, and this favors fragmentation into stars <~1 Msun. Heating of a cloud by accreting low-mass stars within it can prevent fragmentation and allow formation of massive stars, but what properties a cloud must have to form massive stars, and thus where massive stars form in a galaxy, has not yet been determined. Here we show that only clouds with column densities >~ 1 g cm^-2 can avoid fragmentation and form massive stars. This threshold, and the environmental variation of the stellar initial mass function (IMF) that it implies, naturally explain the characteristic column densities of massive star clusters and the difference between the radial profiles of Halpha and UV emission in galactic disks. The existence of a threshold also implies that there should be detectable variations in the IMF with environment within the Galaxy and in the characteristic column densities of massive star clusters between galaxies, and that star formation rates in some galactic environments may have been systematically underestimated.
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