arXiv subjects

M. Rampp

Publications and source records attributed to M. Rampp.

22 records · Page 2Linked to original sources

arXiv2001

Supernova Explosions and Neutron Star Formation

The current picture of the collapse and explosion of massive stars and the formation of neutron stars is reviewed. According to the favored scenario, however by no means proven and undisputed, neutrinos deposit the energy of the explosion in the stellar medium which surrounds the nascent neutron star. Observations, in particular of Supernova~1987A, suggest that mixing processes play an important role in the expanding star, and multi-dimensional simulations show that these are linked to convective instabilities in the immediate vicinity of the neutron star. Convectively enhanced energy transport inside the neutron star can have important consequences for the neutrino emission and thus the neutrino-heating mechanism. This also holds for a suppression of the neutrino interactions at nuclear densities. Multi-dimensional hydrodynamics, general relativity, and a better understanding of the neutrino interactions in neutron star matter may be crucial to resolve the problem that state-of-the-art spherical models do not yield explosions even with a very accurate treatment of neutrino transport by solving the Boltzmann equation.

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arXiv2000

Spherically Symmetric Simulation with Boltzmann Neutrino Transport of Core Collapse and Post-Bounce Evolution of a 15 Solar Mass Star

We present a spherically symmetric, Newtonian core-collapse simulation of a 15 solar mass star with a 1.28 solar mass iron core. The time-, energy-, and angle-dependent transport of electron neutrinos and antineutrinos was treated with a new code which iteratively solves the Boltzmann equation and the equations for neutrino number, energy and momentum to order O(v/c) in the velocity v of the stellar medium. The supernova shock expands to a maximum radius of 350 km instead of only about 240 km as in a comparable calculation with multi-group flux-limited diffusion (MGFLD) by Bruenn, Mezzacappa, & Dineva (1995). This may be explained by stronger neutrino heating due to the more accurate transport in our model. Nevertheless, after 180 ms of expansion the shock finally recedes to a radius around 250 km (compared to about 170 km in the MGFLD run). The effect of an accurate neutrino transport is helpful, but not large enough to cause an explosion of the considered 15 solar mass star. Therefore postshock convection and/or an enhancement of the core neutrino luminosity by convection or reduced neutrino opacities in the neutron star seem necessary for neutrino-driven explosions of such stars. We find an electron fraction Y_e > 0.5 in the neutrino-heated matter, which suggests that the overproduction problem of neutron-rich nuclei with mass numbers around A = 90 in exploding models may be absent when a Boltzmann solver is used for the electron neutrino and antineutrino transport.

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arXiv1997

Simulations of non-axisymmetric rotational core collapse

We report on the first three-dimensional hydrodynamic simulations of secular and dynamical non-axisymmetric instabilities in collapsing, rapidly rotating stellar cores which extend well beyond core bounce. The resulting gravitational radiation has been calculated using the quadrupole approximation. We find that secular instabilities do not occur during the simulated time interval of several 10 ms. Models which become dynamically unstable during core collapse show a strong nonlinear growth of non-axisymmetric instabilities. Both random and coherent large scale initial perturbations eventually give rise to a dominant bar-like deformation ($\exp(\pm imϕ$) with $m=2$). In spite of the pronounced tri-axial deformation of certain parts of the core no considerable enhancement of the gravitational radiation is found. This is due to the fact that rapidly rotating cores re-expand after core bounce on a dynamical time scale before non-axisymmetric instabilities enter the nonlinear regime. Hence, when the core becomes tri-axial, it is no longer very compact.

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arXiv1996

Coalescing neutron stars -- gravitational waves from polytropic models

The dynamics, time evolution of the mass distribution, and gravitational wave signature of coalescing neutron stars described by polytropes are compared with three simulations published previously: (a) ``Run 2'' of Zhuge et al. (1994), (b) ``Model III'' of Shibata et al. (1992), and (c) ``Model A64'' of Ruffert et al. (1996). We aim at studying the differences due to the use of different numerical methods, different implementations of the gravitational wave backreaction, and different equations of state. Comparison (a) confronts the results of our grid-based PPM scheme with those from an SPH code. We found that due to the lower numerical viscosity of the PPM code, the post-merging oscillations and pulsations could be followed for a longer time and lead to larger secondary and tertiary maxima of the gravitational wave luminosity. In case (b) two grid based codes with the same backreaction formalism but differing hydrodynamic integrators and different numerical resolution are compared. Satisfactory agreement of the amplitude of the gravitational wave luminosity is established, although due to the different initial conditions a small time delay develops in the onset of the dynamical instability. In (c) we find that using a polytropic equation of state instead of the high-density equation of state of Lattimer & Swesty (1991) does not change the overall dynamical evolution of the merger and yields agreement of the gravitational wave signature to within 20% accuracy. However, differences of the structure and evolution of the outer layers of the neutron stars are present, which has important implications for questions like mass loss and disk formation during the merging of binary neutron stars.

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