Three-dimensional Simulations of Disk Accretion to an Inclined Dipole. I. Magnetospheric Flows at Different Θ

By M. Romanova, G. V. Ustyugova, Alexander V. Koldoba, J. Wick, R. Lovelace. 2003

195 citations recorded by the source corpus.

We present results of fully three-dimensional MHD simulations of disk accretion to a slowly rotating magnetized star with its dipole moment inclined at an angle Θ to the rotation axis of the disk (which is aligned with the spin axis of the star). The main goal was to investigate the pattern of magnetospheric flow and the disk-star interaction for a variety of inclination angles Θ. We observed that at Θ = 0°, the disk stops at magnetospheric radius rm, and matter flows to the star through axisymmetric funnel flows, as observed in earlier axisymmetric simulations. However, when the dipole moment of the star is inclined, then the flow becomes nonaxisymmetric. The nonaxisymmetry becomes notable at very small inclination angles, Θ ~ 2°-5°. The pattern of magnetospheric flow is different at different Θ. For relatively small angles, Θ ≲ 30°, the densest matter flows to the star mostly in two streams, which follow paths to the closest magnetic pole. The streams typically corotate with the star, but they may precess about the star for Θ ≲ 10°. At intermediate angles, 30° ≲ Θ ≲ 60°, the streams become more complicated and often split into four streams. For even larger angles, Θ ≳ 60°, matter accretes in two streams, but their geometry is different from that of the streams at small Θ. Magnetic braking changes the structure of the inner regions of the disk. It creates a region of lower density (a "gap") for rm ≲ r ≲ 4rm. A ring of higher density forms at r ~ rm for Θ ≲ 30°. For r ≲ (2–3)rm, the azimuthal velocities are sub-Keplerian. The inner region of the disk at r ~ rm is warped. The warping is due to the tendency of matter to corotate with an inclined magnetosphere. The normal of the inner warped part of the disk is close to the magnetic axis of the dipole. The accreting matter brings positive angular momentum to the (slowly rotating) star, tending to spin it up. The corresponding torque NZ depends only weakly on Θ. The angular momentum flux to the star near the star's surface is transported predominantly by the magnetic field; the matter component contributes ~1% of the total flux. The torques NX and NY are also calculated, and these may give a slow precession of the symmetry axis of the star. The angle Θ was fixed in simulations because the timescale of its evolution is much longer than that of the simulations. Results of simulations are important for understanding the nature of classical T Tauri stars, cataclysmic variables, and X-ray pulsars. These stars often show complicated spectral and photometric variability patterns, which may be connected with the structure of magnetospheric flows. The magnetospheric structure of stars with different Θ can give different variability patterns in observed light curves. This can provide information about inclination angles Θ in different stars. A notable result of the present simulations is the formation of multiple streams in the accretion flows near the star for intermediate inclination angles. This may give short-scale, quasi-periodic variability in the light curves of some stars.

Subfields: Astronomy and Astrophysics

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Citations in the corpus

Breaking the chains: hot super-Earth systems from migration and disruption of compact resonant chains (2017, background): “Magnetohydrodynamic simulations of disc–star interaction suggest that a young star with sufficiently strong dipole magnetic field is surrounded by a low-density gas cavity (Romanova et al. 2003; Bouvier et al. 2007; Flock et al. 2017).”
Three-dimensional Simulations of Disk Accretion to an Inclined Dipole. II. Hot Spots and Variability (2004, methodology): “For analysis of the hot spots we use results of our previ- 1 2 ous three-dimensional simulations (Romanova et al. 2003, hereafter - R03), and also perform new simulations at a lower temperature in the disk and at a variety of parameters of the star and the accretion flow.”
Magnetospheric accretion-ejection processes in the classical T Tauri star AA Tauri (2006, background): “–3396 CCD V S. Hodgkin Canarias 28 magnetic fields, leading to a magnetospheric cavity extending over a few stellar radii (e.g. Romanova et al. 2002, 2003).”
Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph (2007, methodology): “These complications are not, however, included in most existing theoretical studies on magnetospheric accretion (e.g., Romanova et al. 2003, 2004).”
Launching of conical winds and axial jets from the disc–magnetosphere boundary: axisymmetric and 3D simulations (2009, methodology): “In earlier simulations of funnel accretion (e.g. R02; Romanova et al. 2003; Long et al. 2005) both viscosity and diffusivity were small and of the same order, and bunching of the magnetic field did not occur.”
“Propeller” Regime of Disk Accretion to Rapidly Rotating Stars (2006, methodology): “Funnel flows were recently investigated numerically using 2D and 3D MHD simulations (Romanova et al. 2002, 2003, 2004a).”
Accretion to magnetized stars through the Rayleigh–Taylor instability: global 3D simulations (2008, methodology): “To model accretion, the ideal MHD equations are solved numerically in three dimensions, using a Godunov-type numerical code, written in a “cubed-sphere” coordinate system rotating with the star (Koldoba et al. 2002; Romanova et al. 2003).”
Magnetospheric accretion on the T Tauri star BP Tauri (2008, background): “The magnetic topology of V2129 Oph is found to be significantly more complex than the conventional dipole used in most theoretical studies (Romanova et al. 2003, 2004); despite being rather weak (0.35 kG), the dipole field component is nonetheless apparently capable of coupling to the accretion…”
Magnetic Fields of Nondegenerate Stars (2009, background): “…locations and geometries are found to depend strongly on the inclination of the magnetosphere with respect to the rotation axis of both the disc and the protostar (e.g., Romanova et al. 2003, 2004) as well as on the large-scale topology of the field (e.g., Gregory et al. 2006; Long et al. 2008).”