Nonlinear Damping of Tides in Stellar and Planetary Systems

菲利普·阿拉斯博士（弗吉尼亚大学）和合作者将研究双星通过潮汐损失的能量。在紧密的双星系统中，引力潮汐的耗散起到同步旋转和使轨道圆化的作用。尽管经过数十年密集的观测和理论研究，流体物体中潮汐耗散的物理机制仍然是一个谜。物理动机，准确的潮汐理论对于理解近距离双星的起源，它们随后的轨道演化以及潮汐加热引起的热演化至关重要。潮汐耗散理论在广泛的系统中得到应用，从系外行星的生存，到包含一颗类太阳恒星的双星的圆化，再到作为强引力波源的致密双星的命运。以前的潮汐耗散研究假设流是层流的，并且大部分的重点放在线性阻尼机制上，如对流区产生的辐射扩散或湍流粘度。这样的研究在解释观测到的系统所需的耗散水平上失败了几个数量级。本课题组在最近的潮汐研究中发现，线性的潮汐流是不稳定的，导致小尺度波浪的生长和潮汐引起的湍流。从本质上讲，层流通常被认为是不存在的类型的情况下，它被使用。这种由潮汐引起的湍流产生的非线性阻尼为传统的线性阻尼机制提供了一种有希望的替代方案。作者将发展非线性潮汐理论，研究每一种感兴趣的恒星或行星结构，线性潮汐流的稳定性，潮汐引起的湍流，同步和循环率，以及深度相关的内部加热。他们对潮汐耗散计算的主要应用是近距离气体巨星系外行星、太阳型双星以及包含白矮星或中子星的激励双星。由于潮汐在近距离双星中无处不在的重要性，这项计划中的研究将对天体物理学中的各种问题产生影响。它将揭示太阳系外行星系统和双星系统演化中长期存在的谜题，并将阐明潮汐加热对热木星内部结构的影响。对近距离白矮星和中子星双星的研究将决定潮汐是否能改变这些系统的轨道演化，这对电磁和引力波测量都有重要意义。这个项目包括培养一名博士后和一名研究生。该团队将向天文学界和公众传播他们的研究结果。

Dr. Philip Arras (University of Virginia) and collaborators will investigate the energy lost by binary stars via tides. Dissipation of gravitational tides in close binary systems acts to synchronize the rotation and circularize the orbit. Despite decades of intense observational and theoretical study, the physical mechanism for tidal dissipation in fluid objects remains a mystery. Physically motivated, accurate theories of tides are crucial to understand the origin of close binaries, their subsequent orbital evolution, and thermal evolution due to tidal heating. Theories of tidal dissipation find application in a broad range of systems, from the survival of exoplanets, to the circularization of binaries containing a Sun-like star, to the fate of compact binaries which are strong gravitational wave sources.Previous investigations of tidal dissipation assumed the flow is laminar and much of the focus was on linear damping mechanisms, such as radiative diffusion or turbulent viscosity arising in convection zones. Such studies fail by orders of magnitude to produce the level of dissipation needed to explain the observed systems. In recent studies of tides by this research team, it was discovered that the linear tidal flow is unstable, leading to the growth of small scale waves and tidally-induced turbulence. In essence, the laminar flow often assumed does not exist for the types of situations in which it is employed. Nonlinear damping from this tidally-induced turbulence provides a promising alternative to conventional linear damping mechanisms.The authors will develop the theory of nonlinear tides by studying, for each type of stellar or planetary structure of interest, the stability of linear tidal flow, tidally induced turbulence, synchronization and circularization rates, and depth-dependent internal heating. The primary applications of their calculations on tidal dissipation are to close-in gas giant extrasolar planets, solar-type binaries, and inspiraling binaries containing white dwarfs or neutron stars.The planned research will have impact on a wide variety of problems in astrophysics, due to the ubiquitous importance of tides in close binaries. It will shed light on longstanding puzzles in the evolution of extrasolar planetary systems and stellar binaries, and will elucidate the impact of tidal heating on the internal structure of Hot Jupiters. The work on close white dwarf and neutron star binaries will determine if tides can alter the orbital evolution of these systems, which has important implications for both electromagnetic and gravitational wave measurements. This project involves training of a postdoc and a graduate student. The team will disseminate the results from their research to the astronomical community and to general public.