Collapsars - Gamma-ray Bursts and Explosions in “failed Supernovae”

Using a two-dimensional hydrodynamics code (PROMETHEUS), we explore the continued evolution of rotating helium stars, Mα >∼10 M⊙ , whose iron core collapse does not produce a successful outgoing shock, but instead forms a black hole of 2 3 M ⊙ . The model explored in greatest detail is the 14 M ⊙ helium core of a 35 M ⊙ main sequence star. The outcome is sensitive to the angular momentum. For j16 ≡ j/(10 16 cm s)<∼ 3, material falls into the black hole almost uninhibited. No outflows are expected. For j16 >∼20, the infalling matter is halted by centrifugal force outside 1000 km where neutrino losses are negligible. The equatorial accretion rate is very low and explosive oxygen burning may power a weak equatorial explosion. For 3 ∼< j16 ∼< 20, however, a reasonable value for such stars, a compact disk forms at a radius where the gravitational binding energy can be efficiently radiated as neutrinos. These are the best candidates for producing gamma-ray bursts (GRBs). Here we study the formation of such a disk, the associated flow patterns, and the accretion rate for disk viscosity parameter, α ≈ 0.001 and 0.1. Infall along the rotational axis is initially uninhibited and an evacuated channel opens during the first few seconds. Meanwhile the black hole is spun up by the accretion (to a ≈ 0.9) and energy is dissipated in the disk by magneto-hydrodynamical (MHD) processes and radiated by neutrinos. For the α = 0.1 model, appreciable energetic outflows develop in cones with polar angle about 30 45 degrees. These outflows, powered by dissipation in the disk, have energy up to a few times 10 erg, mass ∼ 1M ⊙ , and are rich in Ni. They constitute a supernova-like explosion by themselves. Meanwhile accretion through the disk is maintained for at least 20 s, but is time variable (±30%) because of hydrodynamical instabilities at the outer edge in a region where nuclei are experiencing photodisintegration. Because the efficiency of neutrino energy deposition is sensitive to the accretion rate, this instability leads to highly variable energy deposition in the polar regions. Some of this variability, which has significant power at 50 ms and overtones, may persist in the time structure of the burst. During the time followed, the average accretion