Volume 32, Number 2, 1999
ORNL Shines in the World's Astrophysics CommunityOne of the fastest-growing areas in ORNL's Physics Division is the nuclear astrophysics program, which has already become a shining star in the nation's astrophysics community. Five of its members have won major national awards. In 1998 Tony Mezzacappa learned that he had won the Presidential Early Career Award for Scientists and Engineers for his work on the core-collapse supernova explosion mechanism.1 In 1996, this same award was presented to Michael Smith, who heads the Physics Division's Experimental Nuclear Astrophysics Program. In 1997, the award was won by David Dean of the Physics Division, who spends part of his time working on the nuclear theory aspect of the supernova problem. Earlier in 1998 two other Laboratory astrophysicists, Klaus Guber and Paul Koehler, received the Nova Award from Lockheed Martin Corporation for their outstanding research.
ORNL has become a magnet for outstanding theoretical and experimental astrophysicists for two reasons: The Laboratory has strong computational capabilities, and data of astrophysical significance are being obtained at our Holifield Radioactive Ion Beam Facility (HRIBF) and Oak Ridge Electron Linear Accelerator (ORELA).
Mezzacappa and the computational astrophysicists who work with him are among seven groups worldwide modeling core-collapse supernovas, spectacular stellar explosions that produce and disseminate elements such as carbon, nitrogen, and oxygen, which are responsible for life on the earth (see cover). In addition to Mezzacappa, the ORNL group includes Mike Strayer, head of the Physics Division's Theoretical and Computational Physics Section, part-time staff member and University of Tennessee (UT) professor Mike Guidry, postdoctoral scientist Raphael Hix, and UT graduate student Bronson Messer.
These groups are modeling stars greater than 10 times the mass of the sun to predict whether they will explode like Supernova 1987A. The problem is that the calculations of the various groups do not always agree that a star of a certain size with certain characteristics will explode, so efforts are under way to pin down the remaining details of the mechanism. A core-collapse supernova explosion is thought to be caused by a shock wave that results when the star's hot iron core shrinks, compressing its subatomic particles to the point where they repel each other and force the core to rebound.
Astrophysicists believe that the shock wave stalls while trying to propagate from the stellar core through the outer layers of the star and that the shock wave is reenergized by neutrino heating. Neutrinos are particles with no charge and infinitesimally small mass that interact very weakly with matter. Neutrinos of all "flavors," or types, emerge from the proto-neutron star that forms at the center of the explosion.
Mezzacappa says that this central object is like a neutrino "light bulb" radiating heat at the staggering rate of 1045 watts. It is believed, in fact, that these neutrinos power the supernova explosion. Current multidimensional supernova modeling has also uncovered the potential role played by convection—transfer of heat by the circulation of the core's proton-neutron fluid—in aiding this shock revival process.
The ORNL supernova effort leads the field of neutrino transport modeling in both one-dimensional and multidimensional supernova simulations. The most recent ORNL work has underscored the need for more realistic multidimensional simulations of neutrino transport using massively parallel computers.
The ORNL Physics Division also carries out a vigorous program of experimental research in nuclear astrophysics at the HRIBF and ORELA. Part of this work is described in this issue in the next article, "Facilitating Science: ORNL Research at User Facilities."
1The other ORNL winner of this award for 1998 was James Lee of ORNL's Chemical Technology Division. He was cited for his "seminal contributions to photosynthesis research and its application to nanofabrication.