Boundary between Stable and Unstable Regimes of Accretion. Ordered and Chaotic Unstable Regimes

TL;DR

This study uses high-resolution 3D MHD simulations to identify the boundary between stable and unstable accretion regimes onto magnetized stars, revealing the key role of the fastness parameter and magnetosphere size in accretion behavior.

Contribution

It provides a detailed analysis of the transition between stable and unstable accretion regimes, highlighting the influence of the fastness parameter and magnetosphere size on accretion patterns.

Findings

Unstable accretion occurs if omega_s < 0.6.

Chaotic hot spots form for stars with small magnetospheres (r_m/R_star < 7).

Ordered unstable tongues emerge at omega_s < 0.45, merging into fewer structures.

Abstract

We present a study of the Rayleigh-Taylor unstable regime of accretion onto rotating magnetized stars in a set of high grid resolution three-dimensional (3D) magnetohydrodynamic (MHD) simulations performed in low-viscosity discs. We find that the boundary between the stable and unstable regimes is determined almost entirely by the fastness parameter omega_s=Omega_star/Omega_K(r_m), where Omega_star is the angular velocity of the star and Omega_K(r_m) is the angular velocity of the Keplerian disc at the disc-magnetosphere boundary r=r_m. We found that accretion is unstable if omega_s < 0.6. Accretion through instabilities is present in stars with different magnetospheric sizes. However, only in stars with relatively small magnetospheres, r_m/R_star < 7, do the unstable tongues produce chaotic hot spots on the stellar surface and irregular light-curves. At even smaller values of the fastness parameter, omega_s < 0.45, multiple irregular tongues merge, forming one or two ordered unstable tongues that rotate with the angular frequency of the inner disc. This transition occurs in stars with even smaller magnetospheres, r_m/R_star < 4.2. Most of our simulations were performed at a small tilt of the dipole magnetosphere, Theta=5 degrees, and a small viscosity parameter alpha=0.02. Test simulations at higher alpha values show that many more cases become unstable, and the light-curves become even more irregular. Test simulations at larger tilts of the dipole Theta show that instability is present, however, accretion in two funnel streams dominates if Theta > 15 degrees. The results of these simulations can be applied to accreting magnetized stars with relatively small magnetospheres: Classical T Tauri stars, accreting millisecond X-ray pulsars, and cataclysmics variables.