Doing Physics in the Cores of Globular Star Clusters

质量约为太阳八倍的恒星以白矮星(WD)的形式结束生命。这些恒星已经完成了它们的核进化，将它们的核心氢转化为氦，然后转化为碳和氧。然后恒星收缩，直到其电子的压力(量子力学电子简并压力)变得足够高，使恒星达到引力(想要收缩)和压力(想要膨胀)之间的平衡。这一过程的终点是一颗质量约为太阳一半、半径约为地球半径的恒星。到目前为止，WD已经耗尽了其潜在的核燃料来源，所以它只是将存储的热能辐射到太空中，并随着时间的推移缓慢而可预测地冷却-WDS是很好的时钟。这为物理实验室提供了在地球上无法复制的条件；温度约为1亿度，密度约为水的100万倍，并有一个精确的启动时钟。有了这个几乎是独一无二的实验室，就有可能进行奇异的物理实验。WD的冷却速度在其生命的大部分时间里，WD通过发射光子来冷却，但对于非常年轻和非常热的WD来说，这种冷却速度是可以分解的。在表面温度约30,000K以上，冷却的主要来源是恒星核心中中微子的产生。这些中微子能量很低，很难在粒子加速器中产生和检测。使用哈勃太空望远镜(HST)来获得有史以来最大的热WD样本，我们正在测试WD冷却模型，并推断检验当前的中微子理论是否正确。WD在星团中的扩散在星团中产生的WD从最大质量的恒星演化而来时，向星团核心集中。在这个进化过程中，它们失去了大约一半的质量，质量变得太低，不能如此集中在中心，因此开始缓慢向外扩散。因为我们从恒星的冷却时间知道恒星的年龄，所以我们可以建立星团核心中引力相互作用的扩散常数-这是第一次做到这一点。对产生超新星的恒星质量的测定。作为超新星爆炸的恒星的质量下限是星系演化模型的关键输入。这些恒星产生特定的重元素，这些元素最终被合并到后来的恒星和行星中。我们已经开发出一种独特的方法来确定这一点，方法是在附近星系的星系团中寻找最明亮的WD。WDS周围的行星我们一直在寻找古老星团中围绕WDS的行星系统的证据。一个积极的结果可能预示着宇宙中生命的提早出现。我们一直在使用HST(并将在2018年发射后使用其替代的JWST)，在古老的星团中发现了数千个热Ws，并利用它们进行了许多这样的实验。**

Stars up to about eight times the mass of our Sun end their lives as white dwarf stars (WDs). These stars have completed their nuclear evolution, converting their core of hydrogen into helium and then into carbon and oxygen. The star then contracts until the pressure of its electrons (the quantum mechanical electron degeneracy pressure) becomes high enough for the star to reach a balance between gravity (that wants to contract it) and pressure (that wants to expand it). The end point of this process is a star about half the mass of the Sun and about the radius of the Earth. The WD has depleted its potential source of nuclear fuel by this time, so it simply radiates its stored thermal energy out into space and cools slowly and predictably with time - WDS are good clocks. This provides a physical laboratory with conditions that cannot be reproduced on Earth; temperature about 100 million degrees, density of order of 1 million times that of water and a precise clock to boot. With this almost unique laboratory, it is possible to carry out exotic physical experiments. The rate of cooling of WDs Over most of its life, a WD cools by the emission of photons, but this breaks down for the case of very young and very hot WDs. Above surface temperatures of about 30,000K the dominant source of cooling is the production of neutrinos in the core of the star. These neutrinos are of very low energy and hard to produce and detect in particle accelerators. Using the Hubble Space Telescope (HST) to secure the largest samples of hot WDs ever obtained, we are testing WD cooling models and inferentially examining whether current neutrino theory is correct. Diffusion of WDs in star clusters WDs produced in star clusters are concentrated towards the cluster core as they evolve from its most massive stars. During this evolutionary process, they lose about half their mass, become too low in mass to be so centrally concentrated and thus begin to diffuse slowly outwards. Because we know the age of the star from its cooling time, we can establish the diffusion constant due to gravitational interactions in the cluster core - the first time that this has been accomplished. Determination of the masses of stars that produce supernovae. The lower mass limit of stars that explode as supernovae is a critical input into galaxy evolution models. These stars produce specific heavy elements that are eventually incorporated into later generations of stars and planets. We have developed a unique way of determining this by searching for the most luminous WDs in clusters in a nearby galaxy. Planets around WDs We have been searching for evidence of planetary systems around WDs in ancient star clusters. A positive result here could potentially herald an early rise to life in the universe. We have been using the HST (and will use its replacement JWST after its launch in 2018) discovering thousands of hot WDs in ancient star clusters and exploiting them to carry out a number of these experiments.**