Boundary between stable and unstable regimes of accretion. Ordered and chaotic unstable regimes

• Published in: Monthly notices of the Royal Astronomical Society 2016-07, Vol.459 (3), p.2354-2369
• Main Authors: Blinova, A. A., Romanova, M. M., Lovelace, R. V. E.
• Format: Article
• Language: English
• Subjects: Accretion; Accretion disks; Angular velocity; Boundaries; Chaos theory; Instability; Magnetospheres; Pulsars; Simulation; Star & galaxy formation; Stars; Viscosity; X-ray astronomy
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stw786

We present a new study of the Rayleigh–Taylor unstable regime of accretion on to rotating magnetized stars in a set of high grid resolution three-dimensional magnetohydrodynamic 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 ωs = Ω⋆/ΩK(r m), where Ω⋆ is the angular velocity of the star and ΩK(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 ωs ≲ 0.6. Accretion through instabilities is present in stars with different magnetospheric sizes. However, only in stars with relatively small magnetospheres, r m/R ⋆ ≲ 7, do the unstable tongues produce chaotic hotspots on the stellar surface and irregular light curves. At even smaller values of the fastness parameter, ωs ≲ 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 ⋆ ≲ 4.2. Most of our simulations were performed at a small tilt of the dipole magnetosphere, Θ = 5°, and a small viscosity parameter α = 0.02. Test simulations at higher α values show that many more cases become unstable, and the light curves become even more irregular. Test simulations at larger tilts of the dipole Θ show that instability is present, however, accretion in two funnel streams dominates if Θ ≳ 15°. 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 cataclysmic variables.