Alan C. Calder

 

Core Collapse Supernovae

My dissertation research was on the core collapse supernova mechanism. I performed my dissertation research under the direction of Anthony Mezzacappa at ORNL, and my collaborators included Steve Bruenn at Florida Atlantic University, Mike Strayer and Mike Guidry at ORNL, and Sait Umar at Vanderbilt.

Plot of entropy showing vigorous convection in a supernova simulation. The highest enropy is red, the lowest blue.

Core collapse supernovae are the spectacular explosions that mark the violent deaths of massive stars. These events are the most energetic explosions in the cosmos, releasing energy of order 10^{53} erg at the staggering rate of 10^{45-46} watts. Core collapse supernovae produce and disseminate most of the nuclei found in the Universe. Elements heavier than helium, through the iron group, are synthesized during the course of stellar evolution, and via supernovae, are disseminated into the interstellar medium to be reprocessed later in new stars, solar systems, and other astrophysical systems. Supernovae play a key role in the synthesis of heavy elements. In the neutron-rich ``wind'' that emanates from the hot remnant protoneutron star left behind after the star explodes, trans-iron elements are synthesized by a rapid neutron capture process. Additional nucleosynthesis may occur via the $\nu$-process, during which neutrinos emanating from the cooling protoneutron star cause spallations of light particles, particularly neutrons and protons, from abundant heavier nuclei, producing certain rare isotopes. Supernovae also signal the birth of neutron stars and black holes. These important and enigmatic astrophysical objects form from the cooling postsupernova remnant and are the basic building blocks of other astrophysical systems such as pulsars and x-ray binaries.

Core collapse supernovae occur when the iron core of a massive star collapses due to the force of gravity. Once the density in the core exceeds that of nuclear matter, the core rebounds generating pressure waves that propagate outward. At the sonic point, the point at which the velocity of the infalling material exceeds the velocity of sound, the pressure waves become a shock wave that propagates toward the surface of the iron core. A shock generated by this process, collapse and bounce, lacks the energy needed to overcome dissipation due to iron dissociation and neutrino losses and will stall before reaching this surface. What is needed for the star to explode is a shock reheating/reenergizing mechanism.

Contemporary approaches to the reheating mechanism are based on the idea that neutrinos produced in the core transfer gravitational energy released by core collapse to the cooler outer regions of the star. During the reheating process, core electron neutrinos and antineutrinos radiate from their respective neutrinospheres (the sphere within the core of the star defined by the radius above which the stellar material becomes optically thin to neutrinos), and a fraction of these neutrinos are absorbed by the material immediately behind the shock, thereby adding energy to the shock. This process is believed to be enhanced by convection, which may increase the neutrino luminosities and/or the neutrino heating efficiencies, both of which would enhance shock reheating. If the reheating is successful, the shock gains enough energy to reach the surface of the star, and as a result, the star explodes.

My research into the role of convection in the core collapse supernova process resulted in the development of the computer code EVH-1, a new hydrodynamics code for multidimensional flows that couples Piecewise Parabolic Method hydrodynamics to general (not just polytropic/constant $\gamma$) equations of state, and to radiation (in this case, neutrino) transport. Currently, when the matter in our simulations is in nuclear statistical equilibrium we describe its thermodynamic state using the Lattimer-Swesty equation of state, a well-known nuclear equation of state. The code makes use of a nuclear equation of state, and it has a module that calculates local neutrino heating, cooling, and deleptonization, using, at present, neutrino data from other simulations that implement multigroup flux-limited diffusion (MGFLD) and one-dimensional Lagrangian hydrodynamics. MGFLD simulates with sufficient realism the transport of neutrinos in opaque, semitransparent (neutrinosphere), and transparent regions, obviating the need to patch together transport schemes for optically thick and thin regions or the need to oversimplify the neutrino transport and matter coupling above the neutrinospheres. This research was the first implementation of multigroup transport in the context of multidimensional supernova simulations.

My research focused on two modes of convection, proto-neutron star convection and neutrino-driven convection. Proto-neutron star convection is a type of convection that may occur deep in the core of the star immediately after the formation of the shock wave. This type of convection occurs deep enough in the star that the material is optically thick to neutrinos. Convection in this region may enhance the neutrino luminosity by acting as a dredge of hot, neutrino-rich material, which upon rising and cooling will emit the neutrinos thereby boosting the luminosity. By boosting the luminosity of neutrinos emitted from the core, proto-neutron star convection could have the effect adding additional energy (deposited by the neutrinos) to the shock and thereby increase the likelihood of obtaining an explosion. This type of convection is also thought to act as a seed for other types of convection occuring later and farther out of the star. Neutrino-driven convection occurs farther out in the core of the star. It is large-scale convection that occurs just below the shock. Neutrino-driven convection is thought to boost the neutrino heating efficiency (in the material just below the shock). The effect of this would be to increase the neutrino heating of the shock for a given neutrino luminosity and, again, increase the likelihood of an explosion. Also, the large scale nature of this type of convection is thought to affect the dynamics of the shock front leading an aspherical shock. In the case of proto-neutron star convection, the results of my research were rather dramatic: we discovered that proto-neutron star convection is essentially wiped out by neutrino transport. The full details of our work may be found in the proto-neutron star convection paper that is in the Astrophysical Journal (vol. 493 p. 848, 1998). The preprint version may be obtained from the Los Alamos archive by clicking here. In the case of neutrino-driven convection, vigorous ---even supersonic--- convection is evident in our simulations, but despite this, we do not obtain explosions. The full details of our work are in the neutrino-driven convection paper that is also in the Astrophysical Journal (vol. 495 p. 911, 1998). The preprint version may be obtained here. The figure to the above left shows the entropy profile of a simulation with convection present. Higher entropy material appears red, and lower entropy material appears blue.

Movie! Click here to see the proto-neutron star convection movie.

Movies! Click here to see the neutrino-driven convection movie. Or, click here for a lower resolution version if the first is too big. The two movies differ only in number of frames.

Our implementation of realistic one-dimensional multigroup neutrino transport renders these results particularly troublesome for supernova modelers, and indicates that the final verdict on core collapse supernovae must await simulations that implement multidimensional multigroup neutrino transport and hydrodynamics. These simulations must (1) implement detailed neutrino transport that accurately computes the neutrino luminosities, spectra, and angular distributions emerging from the semi-transparent region in which the radiating neutrinospheres are embedded, (2) implement a numerically nondiffusive, high-order hydrodynamics method able to resolve and evolve turbulent convection, strong shocks, and rotation, and (3) implement (1)--(2) self-consistently to compute the feedback between transport, convection, and rotation.

 

Astrophysical Thermonuclear Flashes
Neutron Star Mergers
Core Collapse Supernovae

 

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Last modified: December 12, 2006