Accretion onto stars with octupole magnetic fields: Matter flow, hot spots and phase shifts

Abstract

Recent measurements of the surface magnetic fields of classical T Tauri stars (CTTSs) and magnetic cataclysmic variables show that their magnetic fields have a complex structure. Investigation of accretion onto such stars requires global three-dimensional (3D) magnetohydrodynamic (MHD) simulations, where the complexity of simulations strongly increases with each higher-order multipole. Previously, we were able to model disc accretion onto stars with magnetic fields described by a superposition of dipole and quadrupole moments. However, in some stars, like CTTS V2129 Oph and BP Tau, the octupolar component is significant and it was necessary to include the next octupolar component. Here, we show results of global 3D MHD simulations of accretion onto stars with superposition of the dipole and octupole fields, where we vary the ratio between components. Simulations show that if octupolar field strongly dominates at the disc-magnetosphere boundary, then matter flows into the ring-like octupolar poles, forming ring-shape spots at the surface of the star above and below equator. The light-curves are complex and may have two peaks per period. In case where the dipole field dominates, matter accretes in two ordered funnel streams towards poles, however the polar spots are meridionally-elongated due to the action of the octupolar component. In the case when the fields are of similar strengths, both, polar and belt-like spots are present. In many cases the light-curves show the evidence of complex fields, excluding the cases of small inclinations angles, where sinusoidal light-curve is observed and 'hides' the information about the field complexity.We also propose new mechanisms of phase shift in stars with complex magnetic fields. We suggest that the phase shifts can be connected with: (1) temporal variation of the star's intrinsic magnetic field and subsequent redistribution of main magnetic poles; (2) variation of the accretion rate, which causes the disc to interact with the magnetic fields associated with different magnetic moments. We use our model to demonstrate these phase shift mechanisms, and we discuss possible applications of these mechanisms to accreting millisecond pulsars and young stars.

Complex Magnetospheric Accretion Flows in the Low Accretor CVSO 1335

“…Lastly, the process responsible for the low-velocity redshifted absorption in this star could be similar to that proposed by Bouvier et al (2003), based on results of numerical simulations (Romanova et al 2012;Miller & Stone 1997), namely an "inflated" magnetosphere, resulting from differential rotation between the star and the disk. No radiative transfer model of such geometry has been applied to the Balmer lines in AA Tau yet, although some progress has been made for modeling the low-velocity blueshifted absorption (Esau et al 2014).…”

“…The two-shell geometry could be reminiscent of a complex structure of the stellar magnetic field that gives rise to complex accretion flows. For example, it could be that the accreting material is the combination of magnetic dipolar and multipolar fields (Long et al 2007(Long et al , 2012. Spectropolarimetric observations of accreting T Tauri stars have shown evidences of higher order magnetic fields in stellar magnetosphere; for example Donati et al (2007Donati et al ( , 2011 showed that both the dipole and octupole components are present in the (K5) T Tauri star V2129 Oph.…”

Accretion, Outflows, and Winds of Magnetized Stars

“…That is why, to demonstrate accretion onto a star with octupolar field, the dipole component has been taken to be very small for demonstration (see Fig. 21; Long et al 2012). In this case, the matter accretion forms two equatorial belts.…”

Accretion, winds and outflows in young stars

“…While the stellar magnetic field is certainly more complex, multipole components of higher order are important on smaller scales only. Magneto-hydrodynamic (MHD) simulations of inclined dipoles by Romanova et al (2004) and later more complex fields have confirmed the theoretical idea of magnetically funneled infall (Long, Romanova & Lovelace 2007;Long, Romanova & Lamb 2012), but also revealed new accretion modes, where the accreting matter pushes the magnetic field lines apart and accretes in the plane of the disk (Kulkarni & Romanova 2008).…”