Department of Astronomy and Astrophysics, University of California, Santa Cruz, California 95064, USA.
Nature. 2014 Sep 25;513(7519):523-5. doi: 10.1038/nature13662. Epub 2014 Aug 31.
The abundances of elements in stars are critical clues to stars' origins. Observed star-to-star variations in logarithmic abundance within an open star cluster--a gravitationally bound ensemble of stars in the Galactic plane--are typically only about 0.01 to 0.05 over many elements, which is noticeably smaller than the variation of about 0.06 to 0.3 seen in the interstellar medium from which the stars form. It is unknown why star clusters are so homogenous, and whether homogeneity should also prevail in regions of lower star formation efficiency that do not produce bound clusters. Here we report simulations that trace the mixing of chemical elements as star-forming clouds assemble and collapse. We show that turbulent mixing during cloud assembly naturally produces a stellar abundance scatter at least five times smaller than that in the gas, which is sufficient to explain the observed chemical homogeneity of stars. Moreover, mixing occurs very early, so that regions with star formation efficiencies of about 10 per cent are nearly as well mixed as those with formation efficiencies of about 50 per cent. This implies that even regions that do not form bound clusters are likely to be well mixed, and improves the prospects of using 'chemical tagging' to reconstruct (via their unique chemical signatures, or tags) star clusters whose constituent stars have become unbound from one another and spread across the Galactic disk.
恒星中元素的丰度是研究恒星起源的关键线索。在银河系平面内的开放星团（恒星通过引力束缚在一起的集合体）中，观测到的恒星之间对数丰度的变化通常仅在许多元素中约为 0.01 到 0.05，明显小于星际介质中约 0.06 到 0.3 的变化，而恒星正是从星际介质中形成的。目前还不清楚为什么星团如此均匀，以及在形成效率较低、不会产生束缚星团的区域是否也应该保持均匀。在这里，我们报告了一些模拟，这些模拟跟踪了恒星形成云体聚集和塌缩时化学元素的混合过程。我们表明，在云体聚集过程中的湍流混合自然会产生至少比气体大五倍的恒星丰度离散度，这足以解释观测到的恒星化学均匀性。此外，混合发生得非常早，因此形成效率约为 10%的区域与形成效率约为 50%的区域几乎一样混合。这意味着，即使是那些没有形成束缚星团的区域也很可能是混合良好的，这提高了通过“化学标记”来重建（通过其独特的化学特征或标记）那些组成恒星已经相互分离并散布在银河系盘面的星团的前景。
