ASCI/Alliances Center for Astrophysical Thermonuclear Flashes Helium Detonations on Neutron Stars M. Zingale, F. X. Timmes, B. Fryxell, D. Q. Lamb, K. Olson, A. C. Calder, L. J. Dursi, P. M. Ricker, R. Rosner, J. W. Truran, H. Tufo, P. MacNeice This work is supported by the Department of Energy under Grant No. B to the Center for Astrophysical thermonuclear Flashes at the University of Chicago. These calculations were performed on the Nirvana Cluster at Los Alamos National Laboratory and an SGI Origin 2000 at Argonne National Laboratory. Fryxell et al., 2000 ApJ, in press Fryxell, B. A. & Woosley, S. E ApJ 258, 733 Zingale et al., 2001 ApJS, in press This work is supported by the Department of Energy under Grant No. B to the Center for Astrophysical thermonuclear Flashes at the University of Chicago. These calculations were performed on the Nirvana Cluster at Los Alamos National Laboratory and an SGI Origin 2000 at Argonne National Laboratory. Fryxell et al., 2000 ApJ, in press Fryxell, B. A. & Woosley, S. E ApJ 258, 733 Zingale et al., 2001 ApJS, in press Fig. 5 The adaptive grid shown at 30  s (left) and 150  s (right). Each blue block contains 8  8 computational zones. Fig. 2 Close up of the contact discontinuity behind the break-out shock at 50  s (density is shown). This interface is Rayleigh-Taylor unstable. Fig. 3 Close up of the region behind the detonation front at 90  s. The first surface wave is breaking. Fig. 4 Density at 68  s, just before the photosphere leaves the top of the grid. Fig. 1 Density (left) and temperature (right) every 15  s. The vertical axis extends through the accreted envelope to a height of 1.5 km. The horizontal axis is a 2 km portion along the surface of the neutron star.  Detonation moves at the Chapman-Jouguet velocity, 1.3  10 9 cm s -1, implying a 3 ms propagation time from pole to pole (see figure 1).  Atmosphere oscillates with a period of ~ 50  s  Photosphere flows rapidly off the top of the grid at 68  s, with velocities suggesting a peak height of 10 km (see figure 4).  A series of surface waves propagate behind the detonation front with a velocity of ~ 1.3  10 9 cm s -1. Finite amplitude shallow water wave theory agrees with this speed (see figure 3).  Detailed analysis available in Zingale et al. (2001)  Detonation moves at the Chapman-Jouguet velocity, 1.3  10 9 cm s -1, implying a 3 ms propagation time from pole to pole (see figure 1).  Atmosphere oscillates with a period of ~ 50  s  Photosphere flows rapidly off the top of the grid at 68  s, with velocities suggesting a peak height of 10 km (see figure 4).  A series of surface waves propagate behind the detonation front with a velocity of ~ 1.3  10 9 cm s -1. Finite amplitude shallow water wave theory agrees with this speed (see figure 3).  Detailed analysis available in Zingale et al. (2001) We present the results of a numerical study of helium detonations on the surfaces of neutron stars. These calculations were performed with the FLASH code (Fryxell et al. 2000), a parallel, adaptive, multidimensional hydrodynamics code. We show two-dimensional, cylindrical geometry (r, z) simulations of the evolution of a detonation as it breaks through the accreted envelope of the neutron star and propagates laterally through the accreted material. We were able to confirm the basic results of the only previous multidimensional simulation of such a helium detonation (Fryxell & Woosley 1982), and extended the calculation to reveal a host of new physical phenomena. Results