| Results from SciDAC's Supernova Project |
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| Figure 1. Four steps from a simulation of supernova accretion shock evolution. |
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| In core-collapse supernovae, neutron stars are born spinning at a rate of dozens of times per second. Current theoretical models hold that the high rotation rate of these radio pulsars is the result of the conservation of angular momentum during the collapse of rotating stellar cores. According to these models, pulsar spin is thus directly correlated with the progenitor star’s rotation. Problematically, however, these models predict neutron star rotation rates that are only consistent with the fastest known radio pulsars. |
| Researchers Dr. John M. Blondin of North Carolina State University and Dr. Anthony Mezzacappa of ORNL have recently challenged the conventional assumptions about pulsar rotation. In the January 4, 2007 issue of Nature, Dr. Blondin and Dr. Mezzacappa proposed a new explanation for the generation of neutron star spin which, for the first time, matches astronomical observations. Their results are based on simulations run on the Leadership Computing Facility Cray X1E at ORNL as part of the SciDAC TeraScale Supernova Initiative, a multi-institution collaboration between astrophysicists, nuclear physicists, applied mathematicians, and computer scientists headed by Dr. Mezzacappa. |
| During the collapse of a massive star's core, the outward-directed shock wave, meeting the infalling gas of the stellar implosion, stalls at a radius on the order of 100 km. This stalled-shock phase lasts for less than one second, after which an as-yet undetermined mechanism revives the shock wave and triggers the supernova explosion. Two-dimensional simulations have shown that this quasi-steady shock is subject to the stationary accretion shock instability (SASI), a phenomenon first described by Dr. Blondin in 2003. Simulations in two dimensions, however, were not able to take into account the rotation of the accretion flow. Now Dr. Blondin and Dr. Mezzacappa have performed a series of three-dimensional simulations through which they have determined that the SASI is characterized by a non-axisymmetric spiral flow pattern. |
| The research team explains that the spiral SASI can be understood as a growing acoustic wave propagating around the periphery of the region between the proto-neutron star (PNS) and the accretion shock (figure 1). This robust instability creates two strong counter-rotating flows in the vicinity of the PNS (figure 2), fueling the deposition of angular momentum onto the PNS. |
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| Figure 2. Flow vectors elucidate two rotational flows. |
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| Simulations were run using different initial perturbations, both with and without a moderately rotating progenitor star. In every case, the research team found that the SASI spiral mode became dominant. This dominance was achieved more quickly when the progenitor star was rotating. They also found that the spiral flow pattern generated by the distorted accretion shock had a marked effect on the proto-neutron star. The magnitude of the angular momentum deposited onto the PNS was set by the flow pattern of the spiral SASI wave, not by the angular momentum of the infalling gas above the accretion shock. |
| Likewise, the research team found that the net angular momentum deposited onto the PNS as a result of the spiral SASI had a profound effect on the spin rate of the neutron star left behind after the supernova. For a progenitor star that is not rotating or rotating slowly, the SASI—not the progenitor core spin—will be the dominant source of angular momentum in the remnant neutron star, producing spin rates consistent with observational data for pulsars. Dr. Blondin and Dr. Mezzacappa's results demonstrate that progenitor core spin and neutron star spin are not as simply correlated as prevailing stellar evolution models suggest. Their results also indicate that progenitor core rotation rates may actually be significantly lower than currently predicted. |
| According to Dr. Blondin and Dr. Mezzacappa, the final spin rate of a neutron star is determined by how long the SASI spiral mode was dominant during the core collapse. This in turn depends on how soon the spiral SASI began after the creation of the shock wave as well as on how long the stalled accretion shock lasted before the explosion was triggered. Although the research team’s model is only valid for the stalled-shock phase, and three-dimensional models need to be developed which are sufficiently realistic to allow scientists to follow the entire supernova explosion process, Dr. Blondin and Dr. Mezzacappa's results nevertheless confirm the robustness of the spin-up induced by the spiral SASI, and provide the first plausible explanation for the generation of pulsar spin. |
Contributors:
Dr. John Blondin, North Carolina State University; Dr. Anthony Mezzacappa, ORNI. |
Further Reading
J. M. Blondin and A. Mezzacappa. 2007. Pulsar spins from an instability in the accretion shock of supernovae. Nature, 445: 58-60.
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