Flash code: studying astrophysical thermonuclear flashes

33 The Center for Astrophysical Thermonuclear Flashes at the University of Chicago—a collaboration between scientists primarily at Chicago and Argonne National Laboratory—studies the longstanding problem of thermonuclear flashes on the surfaces of compact stars (such as neutron stars and white dwarfs) and in the interior of white dwarfs (such as Type Ia supernovae). Our central computational challenge is the breadth of physical phenomena involved. These range from accretion flow onto the surfaces of these compact stars to shear-flow and Rayleigh-Taylor instabilities on the stellar surfaces, nuclear burning ignition under conditions leading to convection, stellar envelope expansion, and the possible creation of a common-envelope binary star system. Many of the physical phenomena we encounter have counterparts in the terrestrial realm (in more extreme versions): convection and turbulence at huge Reynolds and Rayleigh numbers, convective penetration of stable matter, state equations for highdensity matter, nuclear processing, radiation hydrodynamics, and interface dynamics (including mixing instabilities and burning front propagation). Perhaps the most spectacular phenomenon with no obvious terrestrial counterpart is the interaction of two binary stars when one of them expands during a nova outburst and swallows its companion (see Figure 1). The complexity of the physics we must deal with is a common theme at all five ASCI/ Alliance Center, and the remarkable physical conditions we encounter distinguishes our center. The energy densities, and many of the physical phenomena, are similar to those dealt with in the US Department of Energy Stockpile Stewardship program. The fully ionized plasmas that ignite under astrophysical conditions are at very high temperatures and densities. The physical problems revolve around nuclear ignition, deflagration or detonation, turbulent mixing, and interface dynamics for complex multicomponent fluids. Our center’s task is to develop a code (Flash code) that can describe the physics of these astrophysical phenomena and that uses modern software technology to efficiently use available massively parallel computers. Although the ultimate test of our results is to compare them with astronomical observations FLASH CODE: STUDYING ASTROPHYSICAL THERMONUCLEAR FLASHES