Accretion onto Stars with Octupole Magnetic Fields: Matter Flow, Hot Spots and Phase Shifts

TL;DR

This study uses 3D MHD simulations to explore how complex magnetic fields in stars influence matter flow, hot spot formation, and phase shifts during accretion, revealing diverse accretion geometries and light-curve behaviors.

Contribution

It introduces a comprehensive model of accretion onto stars with combined dipole and octupole magnetic fields, highlighting new mechanisms for phase shifts related to magnetic and accretion variability.

Findings

Octupolar dominance leads to ring-shaped spots and complex light-curves.

Dipolar dominance results in funnel streams and elongated polar spots.

Similar strength fields produce mixed spot patterns and phase shifts.

Abstract

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.