Three-dimensional Simulations of Disk Accretion to an Inclined Dipole. II. Hot Spots and Variability

By M. Romanova, G. V. Ustyugova, Alexander V. Koldoba, R. Lovelace. 2004

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The physics of the hot spots on 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, μ, is inclined relative to its rotation axis, Ω, by an angle Θ (we call this angle the "misalignment angle"), while the disk's rotation axis is parallel to Ω. A sequence of misalignment angles between Θ = 0° and 90° was investigated. The hot spots arise on the stellar surface because of the impact on the surface of magnetically channeled 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 Θ the spots as observed are shaped like a bow curved around the magnetic axis, while at the largest values of Θ the spots are shaped like a bar crossing the magnetic pole. The physical parameters (density, velocity, temperature, matter, 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 values of Θ and inclination angles of the disk, i. They show a range of variability patterns, including curves with one maximum per period (at most angles Θ and i) and curves with two maxima per period (at large Θ and i). At small Θ, the funnel streams may rotate faster or 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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/43eb6bc617bcbd686197e32de26f527454e59feb

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Citations in the corpus

Accretion onto Pre-Main-Sequence Stars (2016, background): “The oscillations have not been observed (Günther et al. 2010); if the accretion stream is inhomogeneous, as suggested by 3D simulation (Romanova et al. 2004), these oscillations would be difficult to detect (Sacco et al. 2010).”
Accretion to magnetized stars through the Rayleigh–Taylor instability: global 3D simulations (2008, methodology): “Also, if the magnetic and rotational axes are not aligned, there will always be some density enhancement near the disk foot-points of the funnel streams (Romanova et al. 2003, 2004).”
Magnetospheric accretion on the T Tauri star BP Tauri (2008, background): “This process is expected to play a key-role in setting the angular momentum evolution of Sun-like protostars (e.g., Königl 1991; Shu et al. 1994; Cameron & Campbell 1993; Romanova et al. 2004) as well as their internal structure.”
“Propeller” Regime of Disk Accretion to Rapidly Rotating Stars (2006, methodology): “Funnel flows were recently investigated numerically using 2D and 3D MHD simulations (Romanova et al. 2002, 2003, 2004a).”
Magnetospheric accretion-ejection processes in the classical T Tauri star AA Tauri (2006, background): “This suggests a more complex structure for the accretion shock, such as a circumpolar ring around the magnetic axis (Mahdavi & Kenyon 1998; Romanova et al. 2004).”
Magnetic Fields of Nondegenerate Stars (2009, background): “…locations and geometries are found to depend strongly on the inclination of the magnetosphere with respect to the rotation axis of both the disc and the protostar (e.g., Romanova et al. 2003, 2004) as well as on the large-scale topology of the field (e.g., Gregory et al. 2006; Long et al. 2008).”
Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph (2007, methodology): “These complications are not, however, included in most existing theoretical studies on magnetospheric accretion (e.g., Romanova et al. 2003, 2004).”
The Magnetic Fields of Classical T Tauri Stars (2007, background): “…been interpreted in terms of magnetically controlled accretion (e.g., Bertout et al. 1988, 1996; Herbst et al. 2001; Alencar et al. 2001) with the magnetic axis inclined to the rotation axis in some cases (e.g., Kenyon et al. 1994; Johns & Basri 1995; Bouvier et al. 1999; Romanova et al. 2004).”
Launching of conical winds and axial jets from the disc–magnetosphere boundary: axisymmetric and 3D simulations (2009, methodology): “If the star rotates rapidly, it will be in the weak propeller regime in which a star spins down, but no outflows are produced; Romanova et al. 2004a).”