X-ray Bursts, Superbursts, and Outbursts from Accreting Neutron Stars: What heats the Interior?

中子星是宇宙中密度最大的天体，自从它们第一次被猜想以来，就一直吸引着理论家和观察家。尽管在过去的40年里，在电磁频谱上进行了大量的观测，但它们内部发生的许多事情仍然令人费解。由于像钱德拉这样的X射线望远镜、实验室的核实验，以及可能在不久的将来像激光干涉仪引力波天文台(LIGO)这样的引力波探测器，中子星物理的知识正在迅速增长。事实上，美国国家科学院的报告《将夸克与宇宙联系起来：新世纪的11个科学问题》将极高密度和极高温度的物质的性质确定为21世纪的11个关键科学问题之一。此外，最近的2010年十年调查《新世界，新地平线》将其确定为恒星和恒星演化的前沿问题之一，“是什么控制了致密恒星残骸的质量、半径和自转？”在这个项目中开发的模型将使现有和未来的观测能够进一步加深我们对致密物质和恒星演化的物理知识。这个项目包括招募下一代科学家和向公众提供信息的活动，并提供为期三年的研究生支持。该项目支持一名本科生帮助运行项目中使用的代码，并使用开源的Mesa恒星演化代码帮助测试、维护和发布教育资源。在本科生的帮助下，PI将使用MESA开发习题集、情节和动画。密歇根州立大学是一所拥有多样化学生人口的大型公共机构，将作为开发这些资源的试验台。特别是，PI将开发问题“模板”，以便学生可以更容易地使用MESA来探索恒星物理。这些工具将发布在MESA网络论坛http://mesastar.org/.上张贴这些工具将使其他教师更容易采用MESA进行课堂教学，并有望激励其他教师也发布他们的结果。在公众宣传方面，PI将把这一奖项任期内的发现纳入密歇根州立大学艾布拉姆斯天文馆的公开演讲中。更严格地说，对于低质量X射线双星中的中子星，中子星吸积富氢或富氦物质，存在着跨越广泛时间尺度的大量与时间有关的现象。在吸积过程中，可以观察到吸积物质失控的热核燃烧，即I型X射线爆发。观测到的数以千计的X射线爆发，更罕见和更高能量的超暴，以及冷却瞬变，都编码了关于致密物质物理的信息，特别是中子星的亚核密度的“壳”。十多年来，中子星地壳的模型一直是三层相互影响：短爆发产生超爆发的燃料，超爆发火山灰形成外壳，地壳中的核反应加热外壳。然而，在过去的几年里，越来越多的证据表明，这幅图画充其量是不完整的，而且存在着额外的、强烈的、浅的-以及尚未确定的热源。该项目将利用最新的核反应物理，包括新发现的中微子冷却机制，生成中子星地壳的全球模型，并利用现有和未来对冷却瞬变、超暴和X射线爆发的观测来限制地壳中额外加热的深度和强度。此外，他们还将预测地壳成分作为密度函数的唯一性，这对于确定中子星的热和磁演化以及其维持质量四极的能力非常重要。

Neutron stars are the densest objects in the Universe and have fascinated theorists and observers since they were first conjectured. Despite a wealth of observations across the electromagnetic spectrum over the last four decades, much of what happens in their deep interior remains inscrutable. Owing to X-ray telescopes such as Chandra, laboratory nuclear experiments, and perhaps in the near future gravitational wave detectors such as the Laser Interferometer Gravitational Wave Observatory (LIGO), knowledge of neutron star physics is growing rapidly. Indeed, the National Academy report, "Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century" identified the nature of matter at exceedingly high density and temperature as one of the eleven critical science questions for the twenty-first century. Moreover, the recent 2010 Decadal Survey "New Worlds, New Horizons" identifies as one of its Frontier questions for Stars and Stellar Evolution, "What controls the mass, radius, and spin of compact stellar remnants?" The models developed in this project will enable existing and future observations to further our knowledge of the physics of dense matter and stellar evolution.This project includes activities to recruit the next generation of scientists and to inform the public and provides support of a graduate student for three years. The project supports an undergraduate to assist with running the codes used in the project, and to assist with testing, maintaining, and posting educational resources using the open-source MESA stellar evolution code. The PI with assistance from the undergraduate will develop problem sets, plots, and animations using MESA. Michigan State is a large public institution with a diverse student population and will serve as a testbed for development of these resources. In particular, the PI will develop problem "templates" so that students can more readily use MESA to explore stellar physics. These tools will be posted on the MESA web forum http://mesastar.org/. Posting these tools will make it easier for other instructors to adopt MESA for classroom instruction and is expected to motivate others to post their results as well. For public outreach, the PI will incorporate discoveries made during the tenure of this award into public talks at Michigan State's Abrams Planetarium.More technically, for neutron stars in low-mass X-ray binaries, in which the neutron star accretes hydrogen- or helium-rich material, there is a wealth of time-dependent phenomena spanning a wide range of timescales. During accretion, runaway thermonuclear burning of the accreted matter is observable as a Type I X-ray burst. The thousands of observed X-ray bursts, the rarer and more energetic superbursts, and the cooling transients all encode information about the physics of dense matter, in particular the sub-nuclear-density "crust" of the neutron star. For over a decade the model of the neutron star's crust has been that the three layers influence each other: the short bursts produce the fuel for superbursts, superburst ashes form the outer crust, and nuclear reactions in the crust heat the envelope. In the last several years, however, evidence has steadily mounted that this picture is at best incomplete, and that additional, strong, shallow---and as yet unidentified---heat sources are present. This project will generate global models of the neutron star crust with the latest nuclear reaction physics, including a newly discovered neutrino cooling mechanism, and use existing and future observations of cooling transients, superbursts, and X-ray bursts to constrain the depth and strength of additional heating in the crust. In addition, they will make predictions for the uniqueness of the crust composition as a function of density, which is important for determining the neutron star's thermal and magnetic evolution as well as its ability to sustain a mass quadrupole.