原始球状星团、X射线双星和宇宙再电离
Primordial Globular Clusters, X-Ray Binaries & Cosmological Reionisation
摘要：
球状星团是致密的恒星系统，典型的年龄约为130亿年，这意味着它们形成于红移z> 6的时候。新形成或原始球状星团中的大质量恒星可能在宇宙再电离（z>~6）时期发挥了重要作用，作为高能中性氢电离紫外光子的来源。我们调查这些恒星在死亡时是否像它们在主序星生命中那样，作为高能X射线光子的来源一样重要。大多数大质量恒星都是以双星的形式形成的，其中相当一部分（约30%）将演化成X射线发光（L_X~10^38 erg/s）的高质量X射线双星（HMXB）。这些源将在z>~6处对X射线背景作出贡献。利用球状星团的蒙特卡罗模型，我们估算了HMXB的总X射线光度。我们比较和对比这与大质量恒星的总紫外线光度在其主要序列的生活。为了进行合理的估计，我们发现星团的热光度在最初的几百万年中达到峰值约为10^42 erg/s，但在大约500万年后，随着大质量恒星从主序星演化出来，它会下降到约10^41 erg/s。从这时起，总的热光度由HMXB主导，并在约5000万年后逐渐下降到约10^40 erg/s。假设一个幂律的光谱能量分布的HMXB，我们计算的中性氢电离每HMXB的有效数量，并表明，HMXB可以作为电离辐射源的大质量恒星一样重要。最后，我们讨论了我们的研究结果的影响，在高红移星系形成和使用球状星团作为探针的再电离的前景。
英文摘要：
Globular clusters are dense stellar systems that have typical ages of ~13 billion years, implying that they formed at redshifts of z>~6. Massive stars in newly formed or primordial globular clusters could have played an important role during the epoch of cosmological reionisation (z>~6) as sources of energetic, neutral hydrogen ionising UV photons. We investigate whether or not these stars could have been as important in death as sources of energetic X-ray photons as they were during their main sequence lives. Most massive stars are expected to form in binaries, and an appreciable fraction of these (as much as ~30%) will evolve into X-ray luminous (L_X~10^38 erg/s) high-mass X-ray binaries (HMXBs). These sources would have made a contribution to the X-ray background at z>~6. Using Monte Carlo models of a globular cluster, we estimate the total X-ray luminosity of a population of HMXBs. We compare and contrast this with the total UV luminosity of the massive stars during their main sequence lives. For reasonable estimates, we find that the bolometric luminosity of the cluster peaks at ~10^42 erg/s during the first few million years, but declines to ~10^41 erg/s after ~5 million years as the most massive stars evolve off the main sequence. From this time onwards, the total bolometric luminosity is dominated by HMXBs and falls gradually to ~10^40 erg/s after ~50 million years. Assuming a power-law spectral energy distribution for the HMXBs, we calculate the effective number of neutral hydrogen ionisations per HMXB and show that HMXBs can be as important as sources of ionising radiation as massive stars. Finally we discuss the implications of our results for modelling galaxy formation at high redshift and the prospects of using globular clusters as probes of reionisation.
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