DOESciDAC ReviewOffice of Science
SCIENCE ACCOMPLISHMENTS
Astrophysics: Generation of a Neutron Star Spin Period of 50 ms in a 3D Supernova Simulation
The origin of pulsar (rotating neutron star) spin is still unknown. Neutron stars, and therefore pulsars, are born in core collapse supernovae explosions. Given their role as one of the most important sources of elements in the universe, understanding how such explosions occur is one of the most important unsolved problems in astrophysics. The generation of spin in our 3D modeling of core collapse supernova explosions points to a new mechanism for the origins of pulsar spins. The supernova shock wave is unstable and in 3D, this instability leads to a rotating flow below the shock. This results in the deposition of angular momentum onto the forming neutron star. The spin period we obtain is within the range of observed pulsar spin periods. Previous explanations relied solely on the evolution of the stellar core's initial rotation as the core collapses during the supernova. Consequently, there was an artificial one-to-one mapping between initial stellar rotation and final neutron star spin, artificially constraining the former.

Precision observations of neutron star radii and masses allow neutron stars to serve as laboratories for fundamental nuclear physics not accessible to terrestrial experiments. Observations of pulsar orbital decay by the emission of gravitational waves have provided indirect evidence of such waves, predicted by Einstein's general theory of relativity, further motivating direct searches by LIGO and other gravitational wave observatories for gravitational waves from core collapse supernovae and other sources.

Our 3D results also demonstrate how different the outcomes in 3D and 2D simulations of core collapse supernovae are, demonstrating in turn that full 3D models are required to ascertain the core collapse supernova explosion mechanism and to predict all of the observables associated with such supernovae and their remnants. Our results on neutron star spin could not have been obtained in 2D models, where the imposition of artificial symmetries (axial symmetry) reduces the number of degrees of freedom and thereby restricts the possible simulation outcomes. Nature imposes no such symmetries.
Figure 13. The angular momentum in the 3D stellar core flow below the supernova shock wave is depicted in this image. The outer surface is the shock wave. Evident are two large counter-rotating flows, one just below the shock, shown in gold, and the other in the deeper region above the forming neutron star, shown in green. The angular momentum in the inner region is deposited on the neutron star, spinning it up. This mechanism may be responsible for the birth of pulsars (rotating neutron stars). The simulation shown here was performed by John Blondin (NCSU) on the ORNL Leadership Computing Facility.
Our 3D results further demonstrate the efficacy of scientific computation at the terascale and, ultimately, petascale for scientific discovery. It can be argued easily that discovery in the context of nonlinear, multiphysics applications such as ours and the solution of some of nature's Grand Challenge problems will not be achieved in any other way.
With petascale platforms, our 3D hydrodynamics- only simulations can be advanced significantly, allowing full 3D multiphysics simulations of core collapse supernovae. At this scale, the explosion mechanism for core collapse supernovae will finally be ascertained, as well as the underlying mechanism for neutron star spin and other observables associated with core collapse supernovae and the neutron stars they produce.
Contributor: Dr. John Blondin, North Carolina State University (NCSU), SciDAC Terascale Supernova Initiative

 
Published by IOP Publishing in association with Oak Ridge National Laboratory, for the US Department of Energy, Office of Science. Copyright © 2006 by IOP.