Computational Studies of MHD Accretion Flows

从形成中的新恒星(原恒星)，到被烧毁的恒星的余烬(白矮星、中子星和黑洞)，再到活动星系核(活动星系核，如类星体)，各种各样的天体都被认为有吸积盘围绕着它们。从这些圆盘流入中心物体的气体似乎解释了宇宙中一些最戏剧性的组成部分。它们发出惊人的能量，并在巨大的电磁光谱(从中红外到硬X射线和伽马射线)上辐射出去。然而，我们对这些吸积流的理论理解仍然受到建立必要的磁流体动力学(MHD)和辐射流体力学-ICS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当地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围绕原星、白矮星、中子星和黑洞的高空间、光谱和时间分辨率的吸积流观测相比较。该项目将利用计算方法，首次建立吸积盘与磁化和旋转的中心星相互作用的三维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的理论的基础，但到目前为止，它们还没有从第一原理计算出来。这些模拟将直接与一系列不同的观测结果进行比较，包括对T-Tauri星的磁层漏斗流和吸积激波的光谱观测，以及对T-Tauri星自转速度的观测分布。模型的合成光谱将与早期类型星系中心和星系中心吸积黑洞的观测结果进行比较，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。这些全球计算是迈向星盘相互作用模型的第一步，该模型跨越几十年的半径。所有计算都将使用各种2D和3D MHD计算机代码，在国家超级计算机中心的大规模并行机器上使用大量超级计算机时间分配。该项目的资金由NSF河外天文与宇宙学计划(AST/EXC)提供。*

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).***