If only the neutrinos would cooperate. Now that would make Tony Mezzacappa's job much easier. But those skittish particles present all sorts of problems when you're studying supernovae, the powerful phenomena that end the lives of massive stars. And Dr. Mezzacappa, an adjunct physics professor at UT and leader for theoretical astrophysics in the Physics Division at Oak Ridge National Laboratory, has devoted his career to figuring out exactly what happens when these stars reach their violent end, and why. Dr. Mezzacappa explained that there are two types of supernovae. Type Ia supernovae are produced by a thermonuclear runaway. Core collapse supernovae involve the death of massive stars (8-to-10 times the mass of the sun) and what's left behind. His group focuses on the latter. While these scientific wonders enjoy a lot of attention, only a handful of scientists concentrate on the big bang aspect. "We are among a very few groups in the world who study the explosion mechanism," Dr. Mezzacappa said. While a star can live for millions of years, it dies in a matter of hours. Most stars begin their lives burning hydrogen to helium. In large stars, the helium ashes compress until they start burning to carbon and oxygen, then to silicon and neon, and eventually to iron. An iron core accumulates, but its instability eventually causes it to collapse on itself (in one-tenth of a second). The core splits into an inner core and an outer core. The center achieves supernuclear density, eventually becoming so dense it becomes incompressible. The pressure causes the inner core to rebound, launching a shock wave at the boundary between the inner and the outer core that blows the star apart. These explosions are important, Dr. Mezzacappa explained, because of their role in element synthesis and distribution. If you see the star as an onion, with lots of layers, each layer is identified by its predominant element, e.g., iron at the core, followed by shells (or layers) of silicon, oxygen, carbon, helium, and hydrogen. "These massive stars are element factories," Dr. Mezzacappa said, but they aren't the only source. "They (supernovae) actually synthesize elements on their own." Because the earth's biology is carbon-based, knowing how a supernova works can answer fundamental questions about the building blocks of life. The Supernova Recipe For the first two decades of study, scientists had a difficult time creating supernova models because the shock wave that destroys the star would stall. Dr. Mezzacappa explained that as the wave works its way through the star, it expends a lot of energy breaking up nuclei. To complicate matters, the wave hits a border called the electron-neutrino sphere, at which point neutrinos behind the shock can escape, siphoning off energy and further weakening the shock wave. However, in 1982 James Wilson of Lawrence Livermore National Laboratory came up with a new paradigm to get past the stalling problem. He theorized that once the iron core collapses on itself, the cold inner core and a hot shocked mantle remain. They form the neutrino bulb, which radiates at a staggering 1045 watts. As the mantle cools, it shrinks, emitting neutrinos. Dr. Wilson proposed that these neutrinos might provide enough energy to re-energize the shock wave and complete the explosion. This is the neutrino heating paradigm, which presents an interesting theoretical challenge for Dr. Mezzacappa's research group: the question of neutrino transport. Dr. Mezzacappa explained that for the neutrino heating paradigm to work there has to be a sufficient difference between the pressures (energy) behind the shock wave than before it in order to push everything out and cause the explosion. Despite the fact that they have weak interactions with the subatomic particles (neutrons and protons) behind the shock, the neutrinos carry away sufficient energy to make the neutrino heating paradigm plausible (for example, the total supernova energy is 1051 ergs, while the neutrino energy is 1053 ergs). In simulating supernova explosions, the energy of neutrino transport must be predicted with great accuracy for the model to work. Luckily, this is part of ORNL's signature. "It's how we simulate neutrino transport that in part defines our group," Dr. Mezzacappa said. Physics Professor Michael Guidry explained the importance of the neutrino issue. "The supernova problem is a major issue in astrophysics," he said. "Tony has been a long-time advocate of the idea that a proper treatment of the neutrino behavior during the explosion is central to solving the problem. Results obtained within the collaboration headed by Tony, and by other groups, within the last several years suggest that Tony's intuition was correct: a proper treatment of the neutrino transport is essential." Dr. Mezzacappa explained that how neutrinos break free from the "soup" of the mantle is critical, so their neutrino Boltzmann Kinetic Equations must be solved to simulate this accurately. This is computationally intensive work and so requires some outside help. Fortunately, supernova studies capitalize on two of what Dr. Mezzacappa calls the national lab's greatest strengths: working with large projects and tackling challenging computational problems. "Oak Ridge is an impressive place," he said. "The quality of the science and the work is phenomenal." His group also works with Dr. Jack Dongarra, a UT Distinguished Professor in computer science, as well as other applied mathematicians, computer scientists, nuclear physicists, and visualization groups both at UT and ORNL. "We have a lot of ties with different people both here and at the University," he said. The reinforcements are welcome, because while studies to date using spherical simulations have solved the Boltzmann equations, the models still don't end in a star-ripping explosion. Dr. Mezzacappa offers two explanations for this: either the input physics has to be improved or different models should be used. The "input physics" provides the variables such as how neutrinos are produced and how much electron capture occurs during stellar core collapse. Answers to these questions rely on the interdisciplinary nature of astrophysics because they require nuclear physics computation. "Nuclear physics plays a big role in the cosmos," Dr. Mezzacappa said. The second approach, using different models, also draws on the strengths of different scientific talents. Spherical models have been the weapon of choice for replicating supernovae, but breaking spherical symmetry with convection and using three-dimensional models might just reveal the secrets of the supernova recipe which may involve neutrino heating, convection, rotation, and magnetic fields. Visualization scientists can offer help in putting these models together. The possible explanations for the supernova mystery have prompted a fresh look at theory. While the neutrino-driven supernova is one prospective explanation, a second paradigm emerging is the magnetohydrodynamic (MHD)-driven supernova. In this theory, the pressure of magnetic fields increases, making fluid in the center buoyant. Density is lowered and bi-polar outflows break the star apart. A third idea is that stars are obliterated by a combination of neutrino power and MHD effects. "We need to simulate supernovae in one dimension, two dimensions, three dimensions and investigate all these scenarios," Dr. Mezzacappa said. "The challenge is figuring out how they explode." And even that won't complete the picture. Once the explosion puzzle is solved, numerous fundamental questions would remain, such as whether supernovae are determined by class of stars, or go by a star-by-star basis. Dr. Mezzacappa started this journey in graduate school, and still has lots of work ahead. An Able Army While figuring out how massive stars die is a formidable task, Dr. Mezzacappa has an advantage in that he can capitalize on the strengths of ORNL and the UT Department of Physics and Astronomy. Bronson Messer, the department's 2000 Stelson Fellow, defended his doctoral dissertation in November and is joining the Mezzacappa group as a post-doctoral associate. Funding from the National Science Foundation, NASA, and the Department of Energy have helped Dr. Mezzacappa hire gifted students and post-docs, but he's keeping an eye out for new talent. "We can handle quite a few new students at this point," he said. He added that astrophysics offers a number of interesting dissertation projects, not to mention great opportunities within the national laboratory environment. For example, last June ORNL acquired supercomputers from IBM and Compaq to make the lab a major asset for modeling and simulation research. Such resources are one reason the astrophysics group enjoys a high national profile. "A lot of people know who we are and what we're doing" nationally, Dr. Mezzacappa said. Yet he is quick to dispel the myth that his group is confined to the national laboratory. "This is a university-based operation as well," he said. "We very much have a university involvement." Working closely with people like Dr. Guidry, as well as the department's graduate students, helps him keep a finger on the pulse of the University's astrophysics work and allows him "to blend the best of two worlds together." They have also put together a 16-investigator team for DOE's SciDAC initiative involving eight universities. Most of the people involved are university professors. Future assets for the astrophysics group are the proposed Oak Ridge Laboratory for Neutrino Detectors (ORLaND), and the Spallation Neutron Source (SNS). ORLaND would be a national facility for intermediate neutrino studies. Dr. Mezzacappa said it "could provide really critical checks on theory," for astrophysicists. "The Spallation Neutron Source," he said, "will be the single most intense terrestrial source of neutrinos," which would be a fantastic tool for scientists all over the world. "I hope at some point we start to see people … coming here for the supernova work," Dr. Mezzacappa said. Despite the challenges and rewards of the field, he said there is a real need to get people across different disciplines interested because "there are very few people around the country to bring up the next generation." The broader societal importance of his astrophysics collaborations isn't limited to figuring out how stardust plays into our life on earth. The very tools developed for simulation and modeling can be put to use in nuclear medicine and climate modeling as well. "This is not just about pretty pictures," he said. "Clearly, it's about the science." And for now, the science involves understanding why neutrinos behave the way they do, a challenge Dr. Mezzacappa approaches with optimism and humor. "Those neutrinos," he said laughing, "are going to make me gray." Want to know more? Visit the ORNL Astrophysics Program on the Web at: http://www.phy.ornl.gov/astrophysics/astro.html or contact Dr. Mezzacappa at (865) 574-6113 or mezzacappaa@ornl.gov. Cross Sections, Fall 2000 Issue, Contents Page UT Physics News & Notes Page UT Physics Home Page This page was last updated on January 5, 2001. Please send comments to cal@utk.edu.