Variability of Accreting Magnetized Stars Studied with 3D MHD Simulations and 3D Radiative Transfer

吸积磁化的恒星，如经典的T-Tauri星、激变变星和毫秒X射线脉冲星，在它们的光曲线和光谱中显示出广泛的变异性。在某些情况下，观察到精确的周期性，而在另一些情况下，光曲线表现出随机波动或准周期振荡。理解这些磁化恒星需要全球3D磁流体(MHD)建模。罗曼诺娃博士和她的合作者最近进行的3D和2.5维MHD模拟显示，物质在磁化恒星周围流动的可能路径多种多样，包括通过瑞利-泰勒不稳定性直接吸积的可能性。这些研究表明，磁化恒星可能处于稳定的吸积状态，也可能处于不稳定的吸积状态，具有截然不同的观测特性。在MHD模拟中发现的这一现象和其他有趣的现象可能描述了不同吸积磁化恒星的不同性质或演化阶段。但是，为了将结果与观测结果进行比较，人们需要计算恒星和周围物质的连续谱和线谱。这就需要高水平的辐射传输建模，这也是本项目的主要目标：以3D MHD模拟结果为基础，执行全3D辐射传输建模。这种3D(MHD)+3D(辐射)模型的结果将与不同经典T-Tauri星的观测结果进行比较，这些经典T-Tauri星有光谱和光度学观测数据。为了理解磁化恒星周围的不同现象，将进行一些新的MHD模拟，包括(1)通过不稳定性吸积；(2)外流模拟；(3)研究由未对准的偶极子产生的圆盘振荡和翘曲。预计这项研究将是模拟吸积磁化恒星的重要新步骤。两个现代最先进的程序(3D MHD和辐射转移)将被结合在一起，以主要努力理解磁化恒星的物理。这一结果对于理解从磁棕矮星到中子星的吸积磁化恒星的整个范围都将是有价值的。关于圆盘振荡和翘曲的结果将对理解致密恒星周围的过程很有价值。所开发的方法和数值MHD程序也具有普遍价值，适用于行星科学、地球物理、地球磁层科学和日球层科学等其他科学领域以及工程领域。该项目非常适合培训年轻的科学家--研究生和本科生--他们将学习磁流体动力学和编程，如何并行化代码，以及编写辅助程序。该项目还将支持在伊萨卡科学中心举办一场关于恒星诞生的公开展览，展览以吸积和等离子体物理的作用为特色。

Accreting magnetized stars, such as Classical T Tauri stars, cataclysmic variables, and millisecond X-ray pulsars show a wide variety of variability in their light curves and spectra. In some cases, exact periodicity is observed, while in other cases the light curves show stochastic fluctuations or quasi-periodic oscillations. Understanding these magnetized stars requires global 3D magnetohydrodynamic (MHD) modeling. Recent 3D and 2.5D MHD simulations performed by Dr. Romanova and her collaborators have shown a wide variety of possible paths of matter flow around magnetized stars, including the possibility of direct accretion through the Rayleigh-Taylor instability. These investigations have shown that magnetized stars may be either in the stable or unstable regime of accretion, with strikingly different observational properties. This and other interesting phenomena found in their MHD simulations may describe different properties or stages of evolution of different accreting magnetized stars.However, to compare the results with observations, one needs to calculate continuum and line spectra from the stars and the surrounding matter. This requires high-level radiative transfer modeling, which is the primary goal of this project: to perform full 3D radiative transfer modeling, using the results of the 3D MHD simulations as a base. Results of such 3D (MHD) + 3D (radiative) models will be compared with observations of different Classical T Tauri stars for which spectral and photometric observational data are available. A number of new MHD simulations will be done aimed at understanding different phenomena around magnetized stars, including (1) accretion through instabilities; (2) modeling of outflows; (3) investigation of disk oscillations and warping generated by a misaligned dipole.It is expected that this research will be an important new step in modeling accreting magnetized stars. Two modern state-of-the-art codes (3D MHD and radiative transfer) will be combined in a major effort to understand the physics of magnetized stars. The results will be valuable for understanding the whole range of accreting magnetized stars, from magnetic brown dwarfs to neutron stars. The results on disk oscillations and warping will be valuable for understanding processes around compact stars. The methods and numerical MHD codes developed here also have general value and are applicable in other areas of science such as planetary science, geophysics, earth magnetospheric science and heliospheric science, and in engineering. The project is ideal for training young scientists - graduate and undergraduate students - who will learn magnetohydrodynamics and programming, how to parallelize the codes, and to write auxiliary programs. The project will also support a public exhibition at the Ithaca Sciencenter on the birth of stars featuring the roles of accretion and plasma physics.