Accreting magnetized stars, such as Classical T Tauri stars (CTTSs), cataclysmic variables, and X-ray pulsars may have dynamically important magnetic fields that can disrupt the accretion disk. Prior NSF-supported pioneering simulations have shown that accreting magnetized stars may be in stable or unstable regimes of accretion with different observational properties (ordered or stochastic light-curves). If the disk comes close to the star, it interacts through the boundary layer, where simulations have shown Kelvin-Helmholtz-type instabilities. Although these simulations and associated theoretical analyses show many interesting features which could be compared with observation, there is a need further to investigate the disk-magnetosphere/disk-star interaction, including the possibility of some type of Magneto-Rotational Instability (MRI). This new project includes simulations of the disk-magnetosphere interaction specifically tailored with 2.5D and 3D Godunov-type codes to model MRI-driven accretion in relatively thin disks. Associated theoretical analysis will give a deeper understanding of different types of instabilities at the disk-magnetosphere/disk-star boundaries. Because CTTSs show multiple variability features in rotation timescales, theirs are the photometric and spectral light-curves that will be compared with simulations. Spectrum modeling will use a 3D Monte Carlo radiative transfer code. This study will 1) investigate MRI-driven accretion onto a magnetized star; 2) investigate, theoretically and numerically, different instabilities at the disk-magnetosphere and disk-star boundaries; and 3) compare simulated light-curves and spectra with those observed in different CTTSs.
The results will apply to different magnetized stars, including the CTTSs, cataclysmic variables, accreting brown dwarfs and accreting neutron stars. The research will provide a salutary example of comparing state-of-the-art 2.5D and 3D models of magnetized stars with the best available observations. The project will train undergraduate and graduate students and a postdoctoral research associate in this modern area of multidimensional magneto-hydrodynamics. Results will be included in lectures to undergraduate students, to school students, and to Cornell's public visitors. Animated visualizations have recently been developed and will be used in various talks. The team also supports an exhibit in the Tompkins County Science Museum.
The goals of the research were the following: (1) To investigate the accretion of matter onto rottating magnetized stars from ionized discs due to the magneto-rotational instability (MRI) driven turbulence; (2) To investigate the MRI driven accretion onto a star with a weak magnetic field; (3) To investigate theoretically and with MHD simulations possible instabilities of the disc/magnetosphere boundary; and (4) To compare the light curves and speccttra obtained from our simulations with those observed in different Classical T-Tauri Stars (CTTS). Our main results from this research include the following works: (1) `` MRI-driven accretion on to magnetized stars: axisymmetric MHD simulations,'' 2011, MNRAS, 416, pp. 416-438. We present the first results of a global axisymmetric simulation of accretion on to rotating magnetized stars from a turbulent accretion disc, where the turbulence is driven by the magnetorotational instability (MRI). We observed that the angular momentum is transported outwards by the magnetic stress and accretion rate corresponds to a Shakura-Sunyaev viscosity parameter α≈ 0.01-0.04. The disc is stopped by the magnetic pressure of the magnetosphere, and matter flows on to the star in funnel streams, which usually choose a path along top or bottom side of the magnetosphere. c, and that it slowly expands outwards, driven by the magnetic force. (2) ``MRI-driven accretion on to magnetized stars: global 3D MHD simulations of magnetospheric and boundary layer regimes,'' 2012, MNRAS, 421, pp. 63-77. We discuss results of global three-dimensional magnetohydrodynamic simulations of accretion on to a rotating magnetized star with a tilted dipole magnetic field, where the accretion is driven by the magnetorotational instability (MRI). The simulations show that MRI-driven turbulence develops in the disc, and angular momentum is transported outwards primarily due to the magnetic stress. The turbulent flow is strongly inhomogeneous and the densest matter is in azimuthally stretched turbulent cells. We investigate two regimes of accretion: a magnetospheric regime and a boundary layer (BL) regime. (3) ``Magnetic launching and collimation of jets from the disc-magnetosphere boundary: 2.5D MHD simulations,'' 2012, MNRAS, 420, pp. 2020-2033. We use axisymmetric magnetohydrodynamic simulations to investigate the launching and collimation of jets emerging from the disc-magnetosphere boundary of accreting magnetized stars. Our analysis shows that the matter flows into the jet from the inner edge of the accretion disc. It is magnetically accelerated along field lines extending up from the disc and simultaneously collimated by the magnetic pinch force. (4) ``Spectral variability of classical T Tauri stars accreting in an unstable regime,'' 2013, MNRAS, 431, pp. 2673-2689. Classical T Tauri stars (CTTSs) are variable in different time-scales. One type of variability is possibly connected with the accretion of matter through the Rayleigh-Taylor instability that occurs at the interface between an accretion disc and a stellar magnetosphere. In this regime, matter accretes in several temporarily formed accretion streams or `tongues' which appear in random locations, and produce stochastic photometric and line variability. We use the results of global three-dimensional magnetohydrodynamic simulations of matter flows in both stable and unstable accretion regimes to calculate time-dependent hydrogen line profiles and study their variability behaviours. In the stable regime, some hydrogen lines (e.g. Hβ, Hγ, Hδ, Paβ and Brγ) show a redshifted absorption component only during a fraction of a stellar rotation period, and its occurrence is periodic. However, in the unstable regime, the redshifted absorption component is present rather persistently during a whole stellar rotation cycle, and its strength varies non-periodically. In the stable regime, an ordered accretion funnel stream passes across the line of sight to an observer only once per stellar rotation period while in the unstable regime, several accreting streams/tongues, which are formed randomly, pass across the line of sight to an observer. The latter results in the quasi-stationary appearance of the redshifted absorption despite the strongly unstable nature of the accretion. In the unstable regime, multiple hotspots form on the surface of the star, producing the stochastic light curve with several peaks per rotation period. This study suggests a CTTS that exhibits a stochastic light curve and a stochastic line variability, with a rather persistent redshifted absorption component, may be accreting in the unstable accretion regime.
