Collapsar Disks and Winds

Winds blown from collapsar accretion disks may produce observable stellar explosions independent of any GRB-(and afterglow)-producing jets which may be simultaneously produced. The production of winds is controlled by the accretion disk physics, in particular, the nature of disk cooling via neutrino emission and photo-disintegration of heavy nuclei. These temperature-dependent processes depend on the stellar angular momentum via the depth of the gravitational potential at the Kepler radius where the disk forms. Wind-driven stellar explosions which do not make a GRB (or only a faint one) may occur and constitute a new class of supernova explosion. SN1998bw and 1997ef may be examples. A key feature of collapsar winds is that they are capable of producing the radioactive ^(56)Ni necessary to power a supernova light curve. It is possible to make a GRB in a star without significant production of ^(56)Ni. Such a star would not make an observable supernova and no such component would be expected in the light curve of the optical afterglow

High-Frequency Voronoi Noise Reduced by Smoothed Mesh Motion

We describe a technique for improving the performance of hydrodynamics codes which employ a moving Voronoi mesh. Currently, such codes are susceptible to high-frequency noise produced by rapid adjustments in the grid topology on the smallest scales. The treatment for this grid noise is simple; instead of moving the mesh-generating marker points with the local fluid velocity, this velocity field is smoothed on small scales, so that neighboring marker points generally have similar velocities. We demonstrate significant improvement gained by this adjustment in several code tests relevant to the physics which moving-mesh codes are designed to capture.Comment: MNRAS Accepte