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

Authors: M.M. Romanova, G.V. Ustyugova, A.V. Koldoba,  R.V.E. Lovelace, 

The physics of the hot spotson stellar surfaces and the associated variability of accreting magnetized rotating stars is investigated for the first time using fully three-dimensional magnetohydrodynamic simulations. The magnetic moment of the star is inclined relative to its rotation axis W by an angle Q (we will call this angle the misalignment angle) while the disks rotation axis is parallel to W. A sequence of misalignment angles was investigated,between Q  =0 and 90. The hot spots arise on the stellar surface due to the impact on the surface of magnetically channelled accretion streams. The distribution of different parameters in the hot spots reflects those in the funnel streams near the surface of the star. Typically at small Q the spots are observed to have the shape of a bow which is curved around the magnetic axis,while at largest Q the spots have a shape of a bar, crossing the magnetic pole. The physical parameters (density, velocity, temperature, matter and energy fluxes, etc.) increase toward the central regions of the spots,thus the size of the spots is different at different values of these parameters. At relatively low density and temperature,the spots occupy approximately 10 - 20% of the stellar surface, while at the highest values of these parameters this area may be less than 1% of the area of the star. The size of the spots increases with the accretion rate. Rotation of the star leads to the observed variability of brightness. The light curves were calculated for different Q and inclination angles of the disk i .They show a range of variability patterns, including one maximum-per-period curves (at most of angles Q and i ), and two maximum-per-period curves (at large Q  and i ).At small Q, the funnel streams may rotate faster/slower than the star, and this may lead to quasi-periodic variability of the star. The results are of interest for understanding the variability and quasi-variability of Classical T Tauri Stars,millisecond pulsars and cataclysmic variables.

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Last updated on 29.01.07