Andrew MacFadyen

My current research focuses on the collapse and explosion of massive rotating stars. I am interested in those very massive stars (bigger than 20 solar masses) in which the cores collapse to black holes because the ``delayed'' neutrino-absorption mechanism fails either partially or completely. My thesis research has shown that such stars, ``collapsars,'' are capable of producing extremely energetic accretion-powered explosions which can be the source of classical gamma-ray bursts (GRBs) and asymmetric hyper-energetic supernovae. In rapidly rotating stars, an accretion disk forms as the star collapses into a black hole created by the implosion of its core. Rapid accretion of stellar matter into the hole at a rate of 0.0001 - 0.1 solar masses per second can power a variety of stellar explosions. In the case of prompt black hole formation in hydrogen-stripped stars, classical GRBs of the long duration variety (longer than 5 seconds) can be produced with sufficient energy to explain the most energetic GRBs detected so far (see ``Collapsars - Gamma-Ray Bursts and Explosions in "Failed Supernovae",'' MacFadyen & Woosley, ApJ 524, 262-289 (1999)). Recent observational evidence, both the close association of well-localized GRBs with star forming regions and possible direct links between GRBs and supernovae, supports the collapsar model for long-duration GRBs. Longer GRBs and soft x-ray transients are possible when part of the star falls back onto the central proto-neutron star after a weak supernova. The observational signatures of these explosions are diverse and depend on the beaming of the explosion, the amount of radioactive Nickel-56 produced and mixed into the stellar envelope, the angular momentum of the progenitor star and its radius at core collapse. My research utilizes multi-dimensional hydrodynamical simulations to explore stellar collapse and explosion by various mechanisms -- jets, powered either by a MHD mechanism or neutrino annihilation; accretion disk wind, powered by viscous heating; and explosive nuclear energy release. I am currently exploring the case of collapsars occurring in stars which have not been completely stripped of hydrogen and the ensuing jet-driven explosion of the supergiant envelope. I am also studying the nucleosynthesis and explosive nuclear burning in rotating massive stars and stellar explosions due to winds driven from viscous accretion disks.