Local simulations of the magnetized KH-instability in neutron star mergers

by Miguel Angel Aloy (Univ. of Valencia)

Tuesday 9 Jun 2009, 10:30 → 11:30 Europe/Stockholm

Nordita seminar room 132:028

Global simulations show the growth of Kelvin-Helmholtz instabilities in the contact surface of two merging neutron stars. These have been indentified as the sites of efficient amplification of magnetic fields, although these simulations, due to numerical limitations, were unable to determine the saturation level of the field strength, and, thus, the possible back-reaction onto the flow. We investigate the amplification of weak magnetic fields in Kelvin-Helmholtz-unstable shear flows and the back-reaction of the field onto the flow. We use a high-resolution finite-volume code for ideal MHD to perform 2d and 3d local simulations of hydromagnetic shear flows, both for idealised systems and simplified models of merger flows. In 2d, the magnetic field is amplified on time scales of less than 0.01 ms until it reaches locally equipartition with the kinetic energy. Then, it saturates due to resistive instabilities; it disrupts the Kelvin-Helmholtz-unstable vortex and decelerates the shear flow on a secular time scale. We determine scaling laws of the field amplification with initial field and the grid resolution. In 3d, this hydromagnetic mechanism may be dominated by purely hydrodynamic instabilities, leading to less amplification. We find maximum magnetic fields ∼ 10^{16} G locally and r.m.s. maxima over the whole box ∼ 10^{15} G. These maximum values are assumed only for a short period (< 0.1 ms). In the saturated state of most models, the magnetic field is dominated by its component parallel to the shear flow for rather strong initial fields, while weaker fields tend to lead to a more balanced distribution of field energy among the components. In any case, the velocity and magnetic fields show small-scale features like flux tubes. The magnetic field may be amplified efficiently to very high field strength; the maximum magnetic field energy reached in this process is of the order of the kinetic energy associated to the velocity components transverse to the interface between the two neutron stars. Its dynamic relevance may, nevertheless, be bounded, and, in particular, limited to the shear layer. In particular, the field may not be adequate to produce outflows. Due to the short time scales of the field amplification compared to the dynamical times for launching an outflow (i.e., a few milliseconds), the impact of the field on the ejection of relativistic outflows is probably very small.

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