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DISK ACCRETION TO MAGNETIZED STARS

3D SIMULATIONS OF DISK ACCRETION TO AN INCLINED DIPOLE. HOT SPOTS AND VARIABILITY

[abstract] [plots from the paper] [animation] [full text]

Disk accretion to a rotating star with a misaligned dipole magnetic field has been studied further by three-dimensional MHD simulations. This work focuses on the nature of the “hot spots” formed on the stellar surface due to the impact of two or more funnel streams. We investigated the shape and intensity of the hot spots for different misalignment angles Q between the star’s rotation axis W and its magnetic moment µ. Further, we calculated the light curves due to rotation of the hot spots for different angles i between the observer’s line-of-sight and W. The main results are the following: 

1. For small inclination angles, Q < 30°, the hot spots typically have a shape of a bow which is bent around the magnetic pole. At large inclination angles, Q > 60°, the shape becomes bar-like. Often a spot on a given hemi-sphere splits to form two spots, which reflects the splitting of the funnel stream into two streams. The secondary stream is typically weaker than the main stream so that one spot is much larger than the other. 

2. The density, temperature, matter flux and other parameters increase towards the central regions of the so that the spots are larger at lower temperature/density and smaller at larger temperature / density. They cover about 10 −20% of the area of the star at the density level typical for the external regions of the funnel streams (see Figures 1 and 2). The size of the hot spots increases with the accretion rate.

3. The spots have tendency to be located close to the µW plane. They tend to be located downstream of this plane if a star rotates slowly (i.e., the inner region of the disk and the foot-points of the stream rotate somewhat faster than the star), or upstream, if a star rotates relatively fast. The spots wander around their “favorite” position. The amplitude of wandering is smaller in case of the cooler disk. 

4. The calculated light curves reveal the following features: 
(a). The light curve has one peak per one period of rotation of the star and the shape is approximately sinusoidal. This is typical for small and medium misalignment angles,
Q < 45°, and inclination angles i < 60°
(b). The light curve has two peaks per period of rotation. This is typical for all
Q if inclination angle is large, i >75° ◦. At very large misalignment angles, Q > 60°−70°, the double-peak curve is typical for wide range of inclination angles, i > 30°

5. The variation of the shape and location of the spots will lead to departure from the exact variability and to quasi-variability At small misalignment angles, Q < 30°, the streams (and hot spots) may rotate with velocity different from that of the star  thus leading to quasi-periodic oscillations.

 

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