Warps, bending and density waves excited by rotating magnetized stars: results of global 3D MHD simulations

TL;DR

This paper presents the first 3D MHD simulations of accretion discs around magnetized stars, revealing how star-disc magnetic misalignment excites warps and waves, with implications for young stars and pulsars.

Contribution

It introduces novel global 3D MHD simulations demonstrating wave excitation and warp formation due to star magnetic misalignment, expanding understanding of star-disc interactions.

Findings

Strong one-armed warp when star's magnetosphere corotates with the disc

Warp amplitude depends on misalignment angle, peaking between 15-60 degrees

Different wave phenomena occur if the magnetosphere rotates slower than the disc

Abstract

We report results of the first global three-dimensional magnetohydrodynamic simulations of the waves excited in an accretion disc by a rotating star with a dipole magnetic field misaligned from the star's rotation axis (which is aligned with the disc axis). The main results are the following: (1) If the magnetosphere of the star corotates approximately with the inner disc, then we observe a strong one-armed bending wave (a warp). This warp corotates with the star and has a maximum amplitude between corotation radius and the radius of the vertical resonance. The disc's center of mass can deviate from the equatorial plane up to the distance of z_w\approx 0.1 r. However, the effective height of the warp can be larger, h_w \approx 0.3 r due to the finite thickness of the disc. Stars with a range of misalignment angles excite warps. However, the amplitude of the warps is larger for misalignment angles between 15 and 60 degrees. (2) If the magnetosphere rotates slower, than the inner disc, then a bending wave is excited at the disc-magnetosphere boundary, but does not form a large-scale warp. Instead, high-frequency oscillations become strong at the inner region of the disc. These are (a) trapped density waves which form inside the radius where the disc angular velocity has a maximum, and (b) inner bending waves which appear in the case of accretion through magnetic Raleigh-Taylor instability. These two types of waves are connected with the inner disc and their frequencies will vary with accretion rate. Bending oscillations at lower frequencies are also excited including global oscillations of the disc. In cases where the simulation region is small, slowly-precessing warp forms. Simulations are applicable to young stars, cataclysmic variables, and accreting millisecond pulsars.