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

Abstract

We present results of fully three-dimensional magnetohydrodynamic (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 assumed to be 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 r m , 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 non-axisymmetric. The non-axisymmetry 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 co-rotate 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 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 r m ∼ < r ∼ < 4r m . A ring of higher density forms at r ∼ r m for Θ ∼ < 30 • . For r ∼ < (2 − 3)r m , the azimuthal velocities are sub-Keplerian. The inner region of the disk at r ∼ r m is warped. The warping is due to the tendency of matter to co-rotate with 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 N z 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 N x and N y 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 time scale of its evolution is much longer that 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 ...

Accretion onto stars with octupole magnetic fields: Matter flow, hot spots and phase shifts

“…The correlation of the pulsephase offsets with X-ray flux may be an argument in favor of a "moving spots" model, in which the position of hot spots on the stellar surface depends on the accretion rate (Patruno et al 2009a,b). Spot motions have been observed in 3D MHD simulations (Romanova et al 2003(Romanova et al , 2004) and have been proposed as an explanation of timing noise by Lamb et al (2009a,b). This explanation is a possibility.…”

“…Our 3D MHD model has been described in a series of papers (Koldoba et al 2002;Romanova et al 2003;2004;Kulkarni & Romanova 2005;Long et al 2007Long et al , 2008, where the disc accretion onto stars with dipole and more complex magnetic fields has been investigated. Here, we briefly summarize the different aspects of the model.…”

“…The magnetic field looks like a dipole at all distances. The matter is stopped by the dipole component of the magnetosphere and is channelled to the polar regions in two funnel streams, which is typical for accretion onto a star with a dipole field (Romanova et al 2003). In the disc plane (bottom panel), the matter is stopped by the dipole-like field.…”

“…In one model, the phase shifts are connected with the motion of accretion spots on the surface of the star (e.g., Lamb et al 2009a,b;Patruno et al 2010). Such spot motion has been observed in global 3D simulations of disc-magnetosphere interactions (Romanova et al 2003(Romanova et al , 2004Bachetti et al 2010). In another model, it is suggested that the timing noise can be connected with variation of the pulse shapes (e.g., Poutanen et al 2009).…”

Flow Structure in Magnetic CVs

“…However, since the problem is very complicated, the investigations have been performed either in the frame of simplified models (King 1993;Wynn & King 1995;Wynn et al 1997;King & Wynn 1999;Norton et al 2004;Ikhsanov et al 2004;Norton et al 2008), or in a limited region of the stellar magnetosphere (Koldoba et al 2002;Romanova et al 2003Romanova et al , 2004aRomanova et al , 2004b. Only over the last few years, the authors have managed to develop a comprehensive 3D numerical model to calculate the flow structure in close binaries (Zhilkin & Bisikalo 2009, 2010a, 2010b, 2010c.…”