Supernovae, jets, and collapsars

We continue our study of the possible production of supernovae and a variet y of high-energy transients by black hole formation in massive stars endowe d with rotation : the "collapsar model." The black hole may form either pro mptly, since a successful outgoing shock fails to be launched by the collap sed iron core (collapsar Type I), or, in a mild explosion, by fallback (col lapsar Type II). In the latter case, the inner layers of the star initially move outward but lack adequate momentum to eject all the matter exterior t o the young neutron star. Over a period of minutes to hours, similar to0.1- 5 M-. falls back onto the collapsed remnant, turning it into a black hole a nd establishing an accretion disk. The accretion rate, similar to0.001-0.01 M-. s(-1), is inadequate to produce a jet mediated by neutrino annihilatio n but is similar to what has been invoked in magnetohydrodynamic (MHD) mode ls for gamma-ray bursts (GRBs). This fallback is modeled in detail for two 25 M-. progenitors using two different one-dimensional hydrodynamics codes, one Lagrangian and one Eulerian. The production and consequences of jets a re then explored in both sorts of collapsars. Justification is given for as suming that the jet power is a constant times the mass accretion rate, epsi lon (M) over dot c(2), and the consequences of epsilon = 0.001 and 0.01 are explored. Adopting , an initial collimation half-angle of 10 degrees, the opening of the jet as it propagates through the exploding star is strongly influenced not only by the jet's kinetic energy but also by its initial pre ssure and the stellar structure. Cold jets tend to stay collimated and beco me even more so, sometimes having an angle of only a few degrees when they reach the surface. Jets having higher internal pressure than the stellar ma terial through which they pass, or less initial collimation, spread out and tend to make energetic, asymmetric supernovae accompanied, in helium stars , by weak GRBs. SN 1998bw may have been such an event, and other events hav ing energies between that of ordinary GRBs and GRB 980425 await discovery. In supergiant stars, shock breakout also produces bright X-ray transients t hat might be a diagnostic of the model, but even the most powerful jets (eq uivalent isotropic energy 10(54) ergs) will not produce a GRB in a red supe rgiant. For such Type II supernovae the limiting Lorentz factor is Gamma ap proximate to2. Type II collapsars should be more frequent than Type I and m ay power the most common form of gamma-ray transient in the universe. Howev er, the GRBs seen by BATSE are, for the most part, too brief to be Type II collapsars. Those are still attributed to prompt black hole formation. Even there though, the diverse energies and time structure reflect chiefly the viewing angle and the variable collimation of the jet inside the star, not a highly variable "central engine." Indeed, collapsar-induced transients ma y all have a common total energy in the range 10(51)-10(52) ergs.