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

Cornell University, Ithaca NY

Investigators

Abstract

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.