初始质量函数测定的偏差。II。实多重系统与机会叠加
Biases on initial mass function determinations. II. Real multiple systems and chance superpositions
摘要：
在计算年轻星团的IMF时，我们必须考虑到：（a）大多数大质量恒星都是在多个系统中诞生的;（b）大多数IMF都是从无法分辨这些系统的数据中得出的;（c）如果星团太远，成员之间可能会发生多重叠加。在这篇文章中，我使用数值实验来模拟这些现象对观测到的颜色-星等图的影响，以及从中导出的IMF。真正的多重系统影响观察到的或明显的超巨星MF斜率很小，但可以创建一个明显的超大质量恒星的显着人口。当叠加的恒星数量较少时，偶然叠加只会产生很小的偏差，但是一旦达到一定的数量阈值，它们就会影响观测到的斜率和恒星的表观质量上限。我把这些实验应用到本星系群中两个著名的大质量年轻星系团NGC 3603和R136上。在这两种情况下，我表明，所观察到的人口与质量超过120太阳质量的恒星可以解释的影响，未解决的对象，主要是真正的多个系统的NGC 3603和组合的真实和机会对齐多个系统的R136。因此，太阳或近太阳金属丰度的恒星质量上限的现实情况得到加强，甚至可能低于150太阳质量。NGC 3603中心区域的IMF斜率比Salpeter或Kroupa略平坦，伽玛介于-1.6和-2.0之间，这对不精确的星团距离知识产生的不确定性有重要贡献。R136中心的IMF无法用现有数据测量，但新的HST观测可能会改变这种情况。（节略）
英文摘要：
When calculating IMFs for young clusters, one has to take into account that (a) most massive stars are born in multiple systems (b) most IMFs are derived from data that cannot resolve such systems, and (c) multiple chance superpositions between members are expected to happen if the cluster is too distant. In this article I use numerical experiments to model the consequences of those phenomena on the observed color-magnitude diagrams and the IMFs derived from them. Real multiple systems affect the observed or apparent massive-star MF slope little but can create a significant population of apparently ultramassive stars. Chance superpositions produce only small biases when the number of superimposed stars is low but, once a certain number threshold is reached, they can affect both the observed slope and the apparent stellar upper mass limit. I apply those experiments to two well known massive young clusters in the Local Group, NGC 3603 and R136. In both cases I show that the observed population of stars with masses above 120 solar masses can be explained by the effects of unresolved objects, mostly real multiple systems for NGC 3603 and a combination of real and chance-alignment multiple systems for R136. Therefore, the case for the reality of a stellar upper mass limit at solar or near-solar metallicities is strengthened, with a possible value even lower than 150 solar masses. An IMF slope somewhat flatter than Salpeter or Kroupa with gamma between -1.6 and -2.0 is derived for the central region of NGC 3603, with a significant contribution to the uncertainty arising from the imprecise knowledge of the distance to the cluster. The IMF at the very center of R136 cannot be measured with the currently available data but the situation could change with new HST observations. (abridged)
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