Doing Physics in the Cores of Globular Star Clusters

在球状星团的核心进行物理学研究

• 批准号: RGPIN-2016-03665
• 负责人: Richer, Harvey
• 金额: $ 5.39万
• 依托单位: University of British Columbia
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2018
• 资助国家: 加拿大
• 起止时间: 2018-01-01 至 2019-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=663342
• 关键词: Doing; Physics; Cores; Globular; Star

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.**

质量约为太阳8倍的恒星最终会成为白矮星（WDs）。这些恒星已经完成了它们的核演化，将其核心的氢转化为氦，然后转化为碳和氧。然后恒星收缩，直到它的电子压力（量子力学的电子简并压力）变得足够高，使恒星在引力（想要收缩它）和压力（想要膨胀它）之间达到平衡。这个过程的终点是一颗质量约为太阳一半、半径约为地球的恒星。到这个时候，WDS已经耗尽了其潜在的核燃料来源，所以它只是将储存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。这提供了一个物理实验室，其条件无法在地球上复制；温度大约1亿度，密度是水的100万倍，还有一个精确的时钟。有了这个几乎独一无二的实验室，就有可能进行奇特的物理实验。WDs的冷却速度在其生命的大部分时间里，WDs都是通过光子的发射来冷却的，但对于非常年轻和非常热的WDs来说，这种冷却速度就不行了。在表面温度超过30000 k时，冷却的主要来源是恒星核心产生的中微子。这些中微子的能量非常低，很难在粒子加速器中产生和检测到。利用哈勃太空望远镜（HST）获得了迄今为止获得的最大的热WDs样本，我们正在测试WDs冷却模型，并推论检验当前的中微子理论是否正确。星团中WDs的扩散星团中产生的WDs在从质量最大的恒星演化时向星团核心集中。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，无法集中在一起，因此开始缓慢地向外扩散。因为我们从恒星的冷却时间知道它的年龄，我们可以建立由于星团核心的引力相互作用而产生的扩散常数——这是第一次完成。确定产生超新星的恒星的质量。以超新星形式爆炸的恒星的质量下限是星系演化模型的一个关键输入。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。我们已经开发了一种独特的方法来确定这一点，即在附近星系的星系团中寻找最明亮的WDs。WDs周围的行星我们一直在寻找古老星团中WDs周围行星系统的证据。一个积极的结果可能预示着宇宙中生命的早期出现。我们一直在使用HST（并将在2018年发射后使用它的替代品JWST）在古老的星团中发现数千个热WDs，并利用它们进行了许多这样的实验