Shock formation in accretion columns – a 2D radiative MHD approach

We investigate the effects of radiation and magnetic fields on the formation, structure, and stability of the axisymmetric shock and settling flow produced by accreting gas initially directed towards a non rotating stellar surface from a polar column with free-fall speed. We perform one dimensional reference calculations as well as two dimensional calculations with zero, weak and strong magnetic fields. In the one dimensional case and weak magnetic field case in two dimensions, we verify the existence of the cooling over-stability for low accretion rates, finding that the flow is stabilized if the accretion rate exceeds 5 × 1016 gr s−1 onto a solar mass white dwarf of radius 109 cm. In the two dimensional cases with weak magnetic field (β = magnetic pressure/gas pressure = 0.1) a strong pressure driven latitudinal flow also occurs causing gas to move towards the equator. This strong latitudinal motion is suppressed and the amplitude of short period oscillations considerably reduced in cases we consider in which the magnetic field (hereafter MF) is strong enough (β = 10) to confine the flow to the polar column. This suggests that further investigations are needed in order to understand the short period oscillations in accreting stars with a strong MF.