Publication Date:
2016-05-01
Description:
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 s = * / K ( r m ), where * is the angular velocity of the star and 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 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 * 7, do the unstable tongues produce chaotic hotspots on the stellar surface and irregular light curves. At even smaller values of the fastness parameter, 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 * 4.2. Most of our simulations were performed at a small tilt of the dipole magnetosphere, = 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 show that instability is present, however, accretion in two funnel streams dominates if 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.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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