In regions of star formation, protostars and newborn stars accrete mass from their natal clouds. These clouds are threaded by magnetic fields with a strength characterized by the plasma beta---the ratio of thermal and magnetic pressures. Observations show molecular clouds have beta <= 1, so magnetic fields can play a significant role in the accretion process. We have carried out a numerical study of the effect of large-scale magnetic fields on the rate of accretion onto a uniformly moving point particle from a uniform, non-self-gravitating, isothermal gas. We consider gas moving with sonic Mach numbers of up M ~ 45, magnetic fields that are either parallel, perpendicular, or oriented 45 degrees to the flow, and beta as low as 0.01. Our simulations utilize AMR to obtain high spatial resolution where needed; this also allows the simulation boundaries to be far from the accreting object. Additionally, we show our results are independent of our exact prescription for accreting mass in the sink particle. We give simple expressions for the steady-state accretion rate as a function of beta, M, and field orientation. Using typical molecular clouds values of M ~ 5 and beta ~ 0.04 from the literature, our fits suggest a 0.4 M_Sun star accretes ~ 4*10^{-9} M_Sun/year, almost a factor of two less than accretion rates predicted by hydrodynamic models. This disparity grows to orders of magnitude for stronger fields and lower Mach numbers. We discuss the applicability of these accretion rates versus accretion rates expected from gravitational collapse, and when a steady state is possible. This reduction in Mdot increases the time required to form stars in competitive accretion models, making such models less efficient. In numerical codes, our results should enable accurate subgrid models of sink particles accreting from magnetized media.
Three-dimensional radiation hydrodynamics simulations of wandering intermediate-mass black holes considering the anisotropic radiation and dust sublimation
Cosmological simulations of the growth of supermassive black holes and feedback from active galactic nuclei: method and tests
提出了一种在宇宙学模拟中跟踪超大质量黑洞生长和活动星系核反馈的方法。我们的模型是Springel et al.（2005）模型的一个重大修改版本。由于宇宙学模拟缺乏分辨率和物理模型的多相星际介质，他们往往大大低估了邦迪-霍伊尔吸积率。因此，为了使低质量的BH生长，有必要通过大的特设因子来增加稠密气体中的预测的Bondi-Hoyle速率。我们探讨的物理制度，使用这些因素是合理的，并通过这引入一个新的处方气体吸积。活动星系核的反馈是通过将吸积气体的静止质量能量的一部分热耦合到周围介质中来模拟的。我们描述的实施以及模型的局限性，并激励所有的变化相对于以前的工作。我们调查的鲁棒性的预测宇宙恒星形成的历史，红移零宇宙BH密度，BH标度关系，和星系特定的恒星形成率。我们发现，需要增加预测的吸积率，在文献中的标准程序，是最重要的不确定性来源的自由。我们的模拟表明，超大质量黑洞能够调节其增长，释放一个固定的能量为一个给定的晕质量，独立于假设的AGN反馈的效率，这套标准化的BH标度关系。无论BH种子最初是放置在BH缩放关系的上方还是下方，它们都生长到相同的关系上。AGN反馈有效地抑制了大质量星系中的恒星形成。
Learning the Universe at High Redshifts: Impact of Accretion Modeling on Early Black Hole Growth
University of Virginia（弗吉尼亚大学）；Virginia Institute for Theoretical Astronomy, University of Virginia（弗吉尼亚大学理论天体物理研究所）；Leibniz Institute for Astrophysics Potsdam (AIP)（波茨坦莱布尼茨天体物理研究所）；Department of Astronomy, University of California Berkeley（加州大学伯克利分校天文学系）；Harvard-Smithsonian Center for Astrophysics, Harvard University（哈佛-史密森尼天体物理中心）；Department of Astronomy, Columbia University（哥伦比亚大学天文学系）
A Streamer Driving Misalignment in the Circumtriple Disk of GW Ori
University of Florida（佛罗里达大学）；Columbia University（哥伦比亚大学）；Max-Planck Institute for Astronomy (MPIA)（马克斯·普朗克天文学研究所）；University of Galway（戈尔韦大学）；Queen Mary University of London（伦敦玛丽女王大学）
研究发现 GW Ori 三星系统的流致最外侧尘埃环对准偏差约 3°，可能是其盘结构错位的驱动因素，该流或源于邦迪-霍伊尔吸积，GW Ori 仍与诞生星云保持动力学联系。
