Computational Studies of MHD Accretion Flows

各种各样的物体，从新形成的恒星（原恒星），到燃烧的恒星（白矮星、中子星和黑洞）的灰烬，再到活动的星系核（AGN，如类星体），都被认为周围有吸积盘。从这些圆盘流入到中心物体的气体似乎解释了宇宙中一些最引人注目的组成部分。它们释放出巨大的能量，并将其辐射到一个巨大的电磁波谱（从中红外到硬x射线和伽马射线）。然而，我们对这些吸积流的理论理解仍然受到开发必要的磁流体动力学（MHD）和辐射流体动力学计算机模型所涉及的复杂性的限制。在过去的几年里，我们对控制这种流动的局部物理的理解取得了迅速的进展。现在重要的是研究局部过程，如磁旋转不稳定性（MRI）如何决定全球盘的结构和演化，特别是因为只有全球盘模型可以直接与原恒星、白矮星、中子星和黑洞周围吸积流的高空间、光谱和时间分辨率观测结果进行比较。利用计算方法，该项目将开发第一个与时间相关的吸积盘与磁化旋转中心恒星相互作用的三维MHD模型。这些计算将允许定量测量恒星场进入圆盘的混合率，相互作用区域的大小，施加在恒星上的时间平均扭矩，以及可能形成的任何极帽吸积流的几何和运动学。这些量是磁化恒星如何与吸积盘相互作用理论的基础，但迄今为止，它们还没有从第一性原理计算出来。将对模拟结果与大量不同的观测结果进行直接比较，包括对金牛座T星磁层漏斗流和吸积冲击的光谱观测，以及金牛座T星自转速率的观测分布。模型的合成光谱将与观测到的早期型星系中心和星系中心吸积黑洞的光谱进行比较，而质量吸积率的波动可以与RXTE在x射线双星中观测到的x射线变异性进行比较。这些全局计算是迈向星盘相互作用模型的第一步，这种模型的半径可以跨越几十年。这些计算都将使用各种2D和3D MHD计算机代码来执行，在国家超级计算机中心的大型并行机器上使用大量超级计算机时间。本项目由美国国家科学基金会星系外天文学和宇宙学项目（AST/EXC）资助

AST 0098625StoneA wide variety of objects, ranging from new stars in formation (protostars), to objects which are the cinders of burned out stars (white dwarfs, neutron stars, and black holes), to active galactic nuclei (AGN, such as quasars) are thought to have accretion disks surrounding them. Inflows of gas from these disks onto the central object seem to account for some of the most dramatic components of the Universe. They emit prodigious amounts of power and radiate it over a tremendous swath of the electromagnetic spectrum (from the mid-infrared to hard X-rays and gamma rays). Our theoretical understanding of these accretion flows, however, is still limited by the complexities involved in developing the necessary magnetohydrodynamics (MHD) and radiation hydrodynam-ics computer models. Our understanding of the local physics that control such flows has progressed rapidly in the last few years. It is now important to examine how local processes such as the magnetorotational instability (MRI) determine global disk structure and evolution, especially since only global disk models can be directly compared to high spatial-, spectral-, and time-resolution observations of accretion flows around protostars, white dwarfs, neutron stars, and black holes.Using computational methods, this project will develop the first time-dependent, three-dimensional MHD models of the interaction of an accretion disk with a magnetized and rotating central star. These calculations will allow quantitative measurement of the mixing rate of the stellar field into the disk, the size of the interaction region, the time-averaged torque exerted on the star, and the geometry and kinematics of any polar cap accretion flows that might form. Such quantities are fundamental to the theory of how magnetized stars interact with accretion disks, yet to date they have yet to be calculated from first principles. Direct comparison of the simulations to a large and varied set of observations will be undertaken, including spectroscopic observations of magnetospheric funnel flows and accretion shocks in T Tauri stars, and the observed distribution of the rotation rates in T Tauri stars. Synthetic spectra of the models will be compared to that observed for accreting black holes at the center of early type galaxies and the galactic center, while fluctuations in the mass accretion rate can be compared to X-ray variabil-ity observed by RXTE in X-ray binaries. These global calculations are the first step towards star-disk interaction models which span many decades in radius.The calculations will all be performed with a variety of 2D and 3D MHD computer codes, using large allocations of supercomputer time on massively parallel machines at the national supercomputer centers. Funding for this project was provided by the NSF program for Extragalactic Astronomy & Cosmology (AST/EXC).***