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首页> 外文期刊>The Astrophysical journal >LOW-MASS X-RAY BINARIES IN GLOBULAR CLUSTERS: A NEW METALLICITY EFFECT
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LOW-MASS X-RAY BINARIES IN GLOBULAR CLUSTERS: A NEW METALLICITY EFFECT

机译：球形团簇中的低质量X射线双线性：一种新的金属性效应

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摘要

Globular clusters (GCs) containing bright X-ray sources (L_x > 10~(36) ergs s ~(-1)), commonly associated with low-mass X-ray binaries (LMXBs), are found to be significantly denser and more metal-rich than normal nonX-ray clusters both in the Galaxy and in M31. Within a framework where LMXBs in GCs are generated via tidal captures in high-density clusters and (2 + 1) encounters in low-density globulars, the higher incidence of LMXBs with increasing metallicity is shown to be intrinsic and not just a by-product of other effects. Two possible mechanisms are examined: the first one assumes a dependence of the cluster IMF on metallicity as recently published in the literature. The number of observed LMXBs, more frequently occurring in metal-rich clusters, agrees with the predicted number of NS only if metallicity accounts for a minor contribution to the observed variation of the IMF slope. Other alternatives explored, such as the total variation of the observed IMF slopes is due to (1) just metallicity and (2) the combination of metallicity and position in the Galaxy lead to a clear-cut disagreement with the data. In turn, this result may indicate a flatter dependence of the cluster′s IMF on metallicity than that deduced from observed cluster luminosity functions. The second mechanism assumes that, at fixed cluster density, the rate of tidal captures depends on radius and mass of the capturing star. Based on standard stellar models, stars with higher metal content have wider radii and higher masses, hence the rate of tidal captures increases with increasing metallicity. Moreover, since the fixed binary separation and masses of the two components, metal-rich stars fill more easily the Roche lobe, as their radii are larger, there is an additional " evolutionary " reason to favor a higher incidence of LMXBs in metal-rich clusters. From the order of magnitude computations made, the new effect by itself could explain the observed ratio of 4 between the frequencies of X-ray clusters in the metal-rich and metal-poor groups we observationally determined. However, there is no reason to exclude that both mechanisms can be at work.
机译：包含亮X射线源（L_x> 10〜（36）ergs s〜（-1））的球状星团（GC）通常与低质量X射线双星（LMXB）相关联，并且密度显着更高在银河系和M31中，金属比正常的非X射线簇更富金属。在GC中LMXB通过高密度簇中的潮汐捕获和（2 + 1）在低密度球体中相遇而生成的框架内，随着金属含量的增加，LMXB的发生率较高是内在的，而不仅仅是副产物其他效果。研究了两种可能的机制：第一个假设IMF簇对金属性的依赖性，正如最近在文献中发表的那样。仅在金属性对观察到的IMF斜率变化贡献较小的情况下，观察到的LMXB的数量（通常发生在富含金属的簇中）才与NS的预测数量一致。探索了其他替代方法，例如观测到的IMF斜率的总变化是由于（1）仅是金属性，以及（2）金属性和银河系中位置的组合导致与数据的明显分歧。反过来，该结果可能表明，与从观察到的团簇光度函数得出的结果相比，团簇的IMF对金属性的依赖性更平坦。第二种机制假定，在固定的星团密度下，潮汐捕获的速度取决于捕获星的半径和质量。根据标准恒星模型，具有较高金属含量的恒星具有较宽的半径和较高的质量，因此，潮汐捕获的速率随金属性的增加而增加。此外，由于这两个成分的固定二元分离和质量，富金属星更容易填充罗氏瓣，因为它们的半径更大，因此还有另一个“进化”的理由支持富金属的LMXB发生率更高集群。从数量级的计算中，新的效应本身可以解释我们观察确定的富金属和贫金属组中X射线团簇频率之间观察到的4的比率。但是，没有理由排除这两种机制都可以起作用。

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