arXiv subjects

M. Obergaulinger

Publications and source records attributed to M. Obergaulinger.

23 records · Page 2Linked to original sources

arXiv2009

Semi-global simulations of the magneto-rotational instability in core collapse supernovae

Possible effects of magnetic fields in core collapse supernovae rely on an efficient amplification of the weak pre-collapse fields. It has been suggested that the magneto-rotational instability (MRI) leads to rapid field growth. Although MRI studies exist for accretion discs, the application of their results to core collapse supernovae is inhibited as the physics of supernova cores is substantially different from that of accretion discs. We address the problem of growth and saturation of the MRI by means of semi-global simulations, which combine elements of global and local simulations by taking the presence of global background gradients into account and using a local computational grid. We analyze the dispersion relation of the MRI to identify different regimes of the instability. This analysis is complemented by simulations, where we consider a local computational box rotating at sub-Keplerian velocity, and where we allow for a radial entropy gradient. We identify six regimes of the MRI depending on the ratio of the entropy and angular velocity gradient. Our numerical models confirm the instability criteria and growth rates for all relevant regimes. The MRI grows exponentially within milliseconds the flow and magnetic field geometries being dominated by channel flows. The MRI growth ceases once the channels are disrupted by resistive instabilities (due to finite numerical conductivity), and MHD turbulence sets in. From an analysis of the growth rates of the resistive instabilities, we deduce scaling laws for the termination amplitude of the MRI which agree well with our numerical models. We determine the dependence of the development of coherent flow structures in the saturated state on the aspect ratio of the simulation boxes. [abridged]

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arXiv2008

Kink instabilities in jets from rotating magnetic fields

We have performed 2.5D and 3D simulations of conical jets driven by the rotation of an ordered, large-scale magnetic field in a stratified atmosphere. The simulations cover about three orders of magnitude in distance to capture the centrifugal acceleration as well as the evolution past the Alfven surface. We find that the jets develop kink instabilities, the characteristics of which depend on the velocity profile imposed at the base of the flow. The instabilities are especially pronounced with a rigid rotation profile, which induces a shearless magnetic field. The jet's expansion appears to be limiting the growth of Alfven mode instabilities.

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arXiv2007

Relativistic Outflows in Gamma-Ray Bursts

The possibility that gamma-ray bursts (GRBs) were not isotropic emissions was devised theoretically as a way to ameliorate the huge energetic budget implied by the standard fireball model for these powerful phenomena. However, the mechanism by which after the quasy-isotropic release of a few $10^{50} $erg yields a collimated ejection of plasma could not be satisfactory explained analytically. The reason being that the collimation of an outflow by its progenitor system depends on a very complex and non-linear dynamics. That has made necessary the use of numerical simulations in order to shed some light on the viability of some likely progenitors of GRBs. In this contribution I will review the most relevant features shown by these numerical simulations and how they have been used to validate the collapsar model (for long GRBs) and the model involving the merger of compact binaries (for short GRBs).

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arXiv2006

Axisymmetric simulations of magnetorotational core collapse: Approximate inclusion of general relativistic effects

We continue our investigations of the magnetorotational collapse of stellar cores discussing simulations performed with a modified Newtonian gravitational potential that mimics general relativistic effects. The approximate TOV potential used in our simulations catches several features of fully relativistic simulations quite well. It is able to correctly reproduce the behavior of models which show a qualitative change both of the dynamics and the gravitational wave signal when switching from Newtonian to fully relativistic simulations. If this is not the case, the Newtonian and the approximate TOV models differ quantitatively. The collapse proceeds to higher densities with the approximate TOV potential allowing for a more efficient amplification of the magnetic field by differential rotation. Sufficiently strong magnetic fields brake down the core's rotation and trigger a contraction phase to higher densities. Several models exhibit two different kinds of shock generation. Due to magnetic braking, a first shock wave created during the initial centrifugal bounce does not suffice to eject any mass, and the core continues to collapse to supranuclear densities. Another stronger shock wave is generated during the second bounce as the core exceeds nuclear matter density. The gravitational wave signal of these models does not fit into the standard classification. Instead it belongs to the signal type IV introduced by us in the first paper of this series. This signal type is more frequent for the approximate relativistic potential than for the Newtonian one. Strongly magnetized models emit a substantial fraction of their GW power at very low frequencies. A flat spectrum between 10 Hz and > 100 kHz denotes the generation of a jet-like outflow. [Abstract abbreviated]

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arXiv2006

Axisymmetric simulations of magneto--rotational core collapse: dynamics and gravitational wave signal

We have performed a comprehensive parameter study of the collapse of rotating, strongly magnetized stellar cores in axisymmetry to determine their gravitational wave signature based on the Einstein quadrupole formula. We use a Newtonian explicit magnetohydrodynamic Eulerian code based on the relaxing-TVD method for the solution of the ideal MHD equations, and apply the constraint-transport method to guarantee a divergence--free evolution of the magnetic field. We neglect effects due to neutrino transport and employ a simplified equation of state. The pre--collapse initial models are polytropes in rotational equilibrium with a prescribed degree of differential rotation and rotational energy (~ 1 % of the gravitational energy). The initial magnetic fields are purely poloidal the field strength ranging from 10^10 G to 10^13 G. The evolution of the core, whose collapse is initiated by reducing the gas pressure by a prescribed amount, is followed until a few ten milliseconds past core bounce. The initial magnetic fields are amplified mainly by the differential rotation of the core giving rise to a strong toroidal field component. The poloidal field component grows by compression during collapse, but does not change significantly after core bounce if (abbreviated)

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