标题： 天鹅座X-2：中等质量X射线双星的后代 显示英文标题

标题： Cygnus X-2: the Descendant of an Intermediate-Mass X-Ray Binary

摘要： X射线双星天鹅座X-2（Cyg X-2）最近被证明包含一个次星，其光度和温度远高于低质量次巨星应有的水平。我们进行了详细的双星演化计算，结果表明，如果次星初始质量约为3.5 M_sun，并在其主序星阶段末期（或稍不那么可能的是，在离开主序星之后不久）开始质量转移，那么Cyg X-2目前的演化状态是可以理解的。次星的大部分质量必须在早期快速质量转移阶段从系统中被抛出。在目前的阶段，次星的质量约为0.5 M_sun，具有非简并的氦核。它正在壳层中燃烧氢，质量转移是由燃烧壳层的推进驱动的。因此，Cyg X-2属于一个之前研究较少的中等质量X射线双星（IMXBs）类别。我们认为，目前被归类为低质量X射线双星的X射线双星中，可能有相当一部分是IMXBs的后代，并讨论了一些相关的含义。 显示英文摘要

摘要： The X-ray binary Cygnus X-2 (Cyg X-2) has recently been shown to contain a secondary that is much more luminous and hotter than is appropriate for a low-mass subgiant. We present detailed binary-evolution calculations which demonstrate that the present evolutionary state of Cyg X-2 can be understood if the secondary had an initial mass of around 3.5 M_sun and started to transfer mass near the end of its main-sequence phase (or, somewhat less likely, just after leaving the main sequence). Most of the mass of the secondary must have been ejected from the system during an earlier rapid mass-transfer phase. In the present phase, the secondary has a mass of around 0.5 M_sun with a non-degenerate helium core. It is burning hydrogen in a shell, and mass transfer is driven by the advancement of the burning shell. Cyg X-2 therefore is related to a previously little studied class of intermediate-mass X-ray binaries (IMXBs). We suggest that perhaps a significant fraction of X-ray binaries presently classified as low-mass X-ray binaries may be descendants of IMXBs and discuss some of the implications.