Angular momentum transport and magnetism in stars and planets

恒星和行星中的角动量传输和磁性

• 批准号: 2573723
• 金额: --
• 依托单位: UNIVERSITY OF EXETER
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2021
• 资助国家: 英国
• 起止时间: 2021 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2573723
• 关键词: Angular; momentum; transport; magnetism; stars

When motions occur inside a star or planet, they can transport both heat and angular momentum. Because these motions - whether convective, wave-like, or some mix of the two - are ubiquitous, so is angular momentum transport. As a consequence, different parts of planetary and stellar interiors often rotate at different rates, with profound consequences for the mixing of material, the generation of stellar and planetary magnetism, and the end-states of stellar evolution. The spin rates of massive stellar cores, for example, may influence the nature of the compact remnant that occurs after core collapse, and strongly affect the natal spin rates of black holes, now being probed by gravitational wave astronomy.Recent observations have revealed that our current theoretical understanding of the angular momentum transport - and hence of differential rotation - has some major shortcomings. For example, asteroseismic measurements of the rotation rates of stars have demonstrated that evolved stellar cores rotate much slower than simple theoretical models of the transport would predict. Major puzzles have come from observations of objects in our own Solar System, too: for example, the Juno mission has recently revealed that Jupiter's observed banded zonal flows persist only throughout the outermost few percent of the planet, with a transition to nearly solid-body rotation below this. This has been widely interpreted as arising from magnetic field feedbacks on the flow, with the observed transition to solid-body rotation occurring at roughly the depth where the conductivity becomes high enough for the field to couple to the motion. This is in sharp contrast to the Sun, where conductivity is high throughout and yet a substantial shear is maintained. The different outcomes likely reflect the different regimes of flow speed and rotation rate in the two objects -- but a consistent theory of angular momentum transport in magnetised convection zones that can explain both outcomes in any detail has not yet been forthcoming.In this project, you will use a combination of 3D numerical simulations, analytical theory, and 1D modeling to study the angular momentum transport achieved by flows in stellar and planetary interiors. Along the way, you will study the magnetic fields that are generated by the flows (and which in turn also affect the momentum transport). Your precise role will depend to some extent on your own background and interests. Between us (i.e., you, me, and various collaborators), we will aim to conduct a set of 3D simulations in both local (Cartesian) and global (spherical) geometries, and compare the resulting transport to that envisioned in recently-developed semi-analytical theories. This will involve using massively parallel computers based here in Exeter, and elsewhere. We will use the simulations to test and calibrate the semi-analytical prescriptions, and ultimately attempt to incorporate these into a 1D evolutionary model of structure and evolution. We may also, more speculatively, explore the possibility that the heat and angular momentum transport, and field generation, can be solved for self-consistently, in parallel with the evolutionary calculation, essentially by solving a highly simplified set of fluid equations for a finite number of spatial modes.This project would be most suitable for someone with an interest in astrophysical fluid dynamics, as applied to stars or planets. It will require some level of proficiency with computation, so at least a modest amount of programming experience (and a basic familiarity with Unix-based environments) would be helpful. Some prior familiarity with fluid dynamics/MHD would be great, but is not essential.

当星星或行星内部发生运动时，它们可以传输热量和角动量。因为这些运动--无论是对流运动、波动运动还是两者的混合运动--无处不在，所以角动量输运也是如此。因此，行星和恒星内部的不同部分通常以不同的速度旋转，对物质的混合，恒星和行星磁性的产生以及恒星演化的最终状态产生深远的影响。例如，大质量恒星核心的自旋速率可能会影响核心坍缩后致密残余物的性质，并强烈影响黑洞的纳塔尔自旋速率，这一点现在正被引力波天文学所探索。最近的观测表明，我们目前对角动量输运的理论理解--以及因此对微分旋转的理解--存在一些重大缺陷。例如，对恒星自转速率的星震测量表明，演化的恒星核心的自转速度比简单的理论模型预测的要慢得多。主要的谜团也来自于对我们太阳系中天体的观测：例如，朱诺号使命最近揭示了木星观测到的带状纬向流只存在于行星最外层的百分之几，在这一点之下过渡到近乎固体的旋转。这已被广泛解释为产生的磁场反馈的流动，与所观察到的过渡到固体旋转发生在大致的深度，电导率变得足够高的磁场耦合到运动。这与太阳形成鲜明对比，太阳的电导率很高，但仍保持着相当大的剪切。不同的结果可能反映了两个物体中流速和旋转速率的不同状态-但磁化对流区中角动量传输的一致理论尚未出现，可以详细解释这两种结果。在这个项目中，您将使用3D数值模拟，分析理论，和一维建模，研究恒星和行星内部流动实现的角动量传输。沿着，您将研究由流动产生的磁场（反过来也会影响动量传输）。你的确切角色在某种程度上取决于你自己的背景和兴趣。我们之间（即，你，我，和各种合作者），我们的目标是在局部（笛卡尔）和全球（球形）几何结构中进行一组3D模拟，并将由此产生的传输与最近开发的半解析理论中设想的传输进行比较。这将涉及使用大规模并行计算机在这里埃克塞特，和其他地方。我们将使用模拟来测试和校准半分析处方，并最终尝试将这些纳入结构和进化的一维进化模型。我们也可以更大胆地探索热和角动量输运以及场的产生的可能性，这种可能性可以与演化计算并行地自洽求解，基本上是通过求解一组高度简化的有限数量空间模式的流体方程。这个项目最适合于对天体物理流体动力学感兴趣的人，如应用于恒星或行星。它需要一定程度的计算熟练度，因此至少适度的编程经验（以及对基于Unix的环境的基本熟悉）会很有帮助。先熟悉流体动力学/MHD会很好，但不是必需的。