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     Global 3D Simulations of Accretion through Instabilities

Numerical simulations show that accreting magnetized stars may be in one of two regimes: stable or unstable, with different observational properties (see Romanova, Kulkarni, Lovelace astro-ph, 2007; Kulkarni & Romanova 2007, in prep.)

Stable accretion is characterized by the periodic or almost periodic light-curve, while unstable accretion may result in stochastic light-curve. Instability occurs at small misalignment angles Q and sufficiently high accretion rates.

Accretion through funnel streams in the stable regime. This animation shows accretion in the stable regime through quasi-stationary funnel streams for misalignment angle  Q=15^o as seen in the observer's frame. The green surface is a constant density surface, and red lines are sample magnetic field lines. Funnel streams hit the surface of the star at approximately the same position at all times, creating quasi-stationary hot spots. The rotation axis is at i= 45^o to the line of sight. Animation 1 (avi, 3.4 Mbt).

Accretion through 3D instabilities. This animation shows the temporal evolution of the tongues (green background shows one of density levels) and magnetic field lines (red lines) for Q=5^o in the coordinate system rotating with the star. Grid resolution is N_r x N_x x N_y = 144 x 61 x 61 in each of 6 blocks of the "cubed sphere" grid.  Animation 2 (avi, 5.7 Mbt).

Accretion through 3D instabilities. This animation shows the temporal evolution of the
tongues (seen in the equatorial slice) and magnetic field lines (red lines) for Q=5^o in the coordinate system rotating with the star. The equatorial slice shows the density distribution (the highest density is shown in red, and the lowest in dark-blue). Animation 3 (avi, 5.5 Mbt)

Accretion through 3D instabilities. This animation shows the temporal evolution of the tongues seen in the equatorial slice (the highest density is shown in red, and the lowest in dark-blue). The line  shows modified plasma parameter b=(p + r v²)/(B²/8p)=1.  Animation 4 (avi, 7.2 Mbt).

Periodic rotation of hot spots during stable accretion. This animation shows the hot spots on the surface of a rotating star with misalignment angle  Q=15^o . The colors show the energy density at the surface of the star (the maximum is shown in red, and the minimum in dark blue). The rotation axis is at i= 45^o relative to the line of sight. The position of the spots on the surface of the star is approximately fixed, so that rotation of the star leads to almost periodic light curve. Animation 5 (avi, 4.7 Mbt). 

Hot spots in case of unstable accretion. This animation shows a rotating star with hot spots on its surface for the case of unstable accretion through tongues (Q=15^o). The colors show the energy-density at the surface of the star (the maximum is shown in red, and the minimum in dark blue). The rotation axis is at  i= 45^o relative to the line of sight. The spots change their number and position, giving irregular light curves. Animation 6 (avi, 5.1 Mbt).

       See also stereo animations at http://astrosun2.astro.cornell.edu/us-rus/stereo.htm