This visual rendering of data output from a 3D supernova simulation shows a surface of constant temperature below the supernova shock wave. The green areas show regions of fluid overturn, and the blue areas show regions of fluid inflow or outflow. Combined, the blue and green regions depict the turbulent environment beneath the supernova shock wave. (Figure by Anthony Mezzacappa and Ross Toedte, Oak Ridge National Laboratory, and John Blondin, North Carolina State University)

Anthony Mezzacappa, Oak Ridge National Laboratory

Research Objectives
We will take a major step forward in simulating core collapse supernovae in two and three dimensions with Boltzmann neutrino transport by implementing ray-by-ray Boltzmann transport. Ray-by-ray simulations capture much of the transport realism in multidimensional models by performing independent calculations of the radiation transport along each radial direction. This neglects only contributions from lateral neutrino transport, which will likely only be important below the neutrinospheres in the proto-neutron star, where the neutrinos and matter are strongly coupled and the flow may be highly nonspherical. Nonetheless, ray-by-ray simulations will mark a major advance in multidimensional supernova models, particularly when the flow of neutrinos along each ray is handled using multigroup Boltzmann transport, a much more sophisticated approach than the transport used in past multidimensional supernova simulations. This is an intermediate step towards completing simulations with true multidimensional Boltzmann transport and continues our effort to develop scalable radiation (in our case, neutrino) transport on MPP platforms.

Computational Approach
Our 2D and 3D supernova simulations with ray-by-ray neutrino transport will couple our multidimensional PPM hydrodynamics code, VH-1, with our existing 1D Boltzmann neutrino transport code, BOLTZTRAN. BOLTZTRAN will be used to perform independent transport calculations along each radial ray, allowing us to achieve a high degree of parallelism.

Accomplishments
We have developed the RadHyd framework that will allow us to merge disparate hydrodynamics, transport, and nuclear physics codes in a modular fashion, both for use in the ray-by-ray simulations and for future work. Adaptations made by Calder and Mezzacappa to a prior version of VH-1 have been integrated into the newer, MPI-based massively parallel version of VH-1. In addition, we have added the capability to track nuclear composition, and we have generalized the handling of equations of state. Using the enhanced VH-1module, RadHyd has been extensively validated, in one and two dimensions, against a number of known hydrodynamics test problems. For use in the ray-by-ray simulations, our existing Boltzmann neutrino transport code for spherically symmetric flows, BOLTZTRAN, has been integrated into the RadHyd framework to calculate the neutrino transport, providing an exact transport solution along each ray.

We have tested the combination of BOLTZTRAN and EVH-1 through a series of spherically symmetric core collapse simulations to allow the comparison of RadHyd to our previous results, and have completed our first 2D ray-by-ray simulations.

Significance
Our goal is to understand the mechanism by which core collapse supernovae explode. A signal of the demise of a massive star and the birth of a neutron star or black hole, core collapse supernovae are among the brightest events in the Universe and create many of the chemical elements that make up our solar system. They are the key link in our chain of origins from the Big Bang to the present. To reach this goal we must develop scalable radiation hydrodynamics, which will enable a new class of multidimensional supernova models and will have broad implications for a variety of applications, such as combustion modeling, climate modeling, and nuclear medicine.

Publications
M. Liebendoerfer, A. Mezzacappa, F.-K. Thielemann, O. E. B. Messer, W. R. Hix, and S. W. Bruenn, "Probing the gravitational well: No supernova explosion in spherical symmetry with general relativistic Boltzmann neutrino transport," Phys. Rev. D 63, 103004 (2001).

A. Mezzacappa, M. Liebendoerfer, O. E. B. Messer, W. R. Hix, F.-K. Thielemann, and S. W. Bruenn, "Simulation of the spherically symmetric stellar core collapse, bounce, and postbounce evolution of a 13 solar mass star with Boltzmann neutrino transport, and its implications for the supernova mechanism," Phys. Rev. Lett. 86, 1935 (2001).

J. F. Beacom, R. N. Boyd, and A. Mezzacappa, "Black hole formation in core-collapse supernovae and time-of-flight measurements of the neutrino masses," Phys. Rev. D 63, 073011 (2001).