Beginning with the first numerical simulations conducted by Colgate and White(1966), three decades of supernova modeling have established a basic supernova paradigm. The supernova shock wave--formed when the iron core of a massive star collapses gravitationally and rebounds as the core matter exceeds nuclear densities--stalls in the iron core as a result of enervating losses to nuclear dissociation and neutrinos. The failure of this `prompt'' supernova mechanism sets the stage for a `delayed' mechanism, whereby the shock is reenergized by heating of material by the intense neutrino flux emerging from the neutrinospheres (Wilson 1985; Bethe & Wilson 1985). The neutrinos carry off the binding energy of the proto-neutron star. The neutrino heating is mediated primarily by the absorption of electron neutrinos and antineutrinos on the dissociation-liberated nucleons behind the shock. This neutrino heating is illustrated in Figure 3. The neutrino energy deposition behind the shock depends sensitively not only on the neutrino luminosities but also on the neutrino spectra and angular distributions in the postshock region, necessitating exact multigroup (multi-neutrino energy) Boltzmann neutrino transport or a good approximation. Ten percent variations or less in any of these quantities can make the difference between explosion and failure in supernova models (Janka & Mueller 1996, Burrows & Goshy 1993).
Schematic illustrating the fundamental components during
the neutrino shock revival phase in core collapse supernovae. The region between
the radiating neutrinospheres and the stalled shock splits in two: There is
a net cooling region and a net heating region. The two are separated by the
`gain' radius.
This past decade has also seen the emergence of multidimensional supernova models, which have investigated the role convection, rotation, and magnetic fields may play in the explosion (Herant et al. 1994, Burrows et al. 1995, Janka & Mueller 1996, Mezzacappa et al. 1998ab, Fryer & Heger 2000, Khokhlov et al. 1999). Convection below the neutrinospheres may significantly boost the neutrino luminosities by dredging hotter, lepton-rich matter from deep within the core, and convection directly beneath the supernova shock may significantly boost the shock radius and neutrino heating efficiency, as material heated from below by neutrinos convects upward and does work on the shock. Convection behind the shock is evident in the figure below.
The inclusion of rotation in supernova models will no doubt qualitatively and quantitatively alter the hydrodynamic flow during core collapse and the onset of explosion when realistic three-dimensional models are finally achieved, and together with magnetic fields, is integral to what may be an alternative explosion mechanism--an MHD driven rather than a neutrino driven mechanism--that has been considered in this context (LeBlanc & Wilson 1970, Symbalisty 1984, Khokhlov et al. 1999). Rotation and magnetic fields also play an important role in collapsar models, in which a rotating black hole forms at the center of a massive star and becomes the central engine powering bipolar outflows either by an MHD mechanism, neutrino heating, or both. The collapsar model is a possible supernova mechanism for very massive stars (M > 30 solar masses) and was developed in part to explain the supernova/gamma ray burst association SN1998bw/GRB 980425(Galama et al. 1998). To account for its brightness and high velocities, spherically symmetric models of SN1998bw must invoke kinetic energies one order of magnitude larger than expected from ``ordinary'' core collapse supernovae. In such a model, SN1998bw is a ``hypernova.'' However, it has been shown that, with jet-like supernova explosions and relativistic beaming, observations of SN1998bw can be explained in the context of standard core collapse supernova models (Wheeler et al. 2000).
Three panels showing the development of neutrino-driven
convection behind the supernova shock wave in a simulation carried out by Mezzacappa
et al. (1998b). Hot, expanding upflows (red) reach the shock, exert pressure, and
move the shock outward and distort it from sphericity. Denser, cooler, fingerlike
(yellow) downflows replace the material that has risen from below.
Whatever mix of neutrino heating, convection, rotation, and magnetic fields ultimately gives rise to an explosion, one thing is certain: all of these phenomena take place in the intense general relativistic gravitational field of the proto-neutron star. Simulations carried out by Bruenn et al. (2001), Mezzacappa et al. (2001), and Liebendoerfer et al. (2001) comparing Newtonian and general relativistic simulations of stellar core collapse, bounce, and postbounce evolution clearly demonstrate that Newtonian simulations are not realistic. Therefore, ultimately, multidimensional simulations of core collapse supernovae that include realistic multidimensional, multifrequency neutrino transport coupled with multidimensional hydrodynamics or magnetohydrodynamics must also solve the Einstein equations for the gravitational field. This is particularly true if detailed gravitational wave forms are to be computed for comparison with future detections by LIGO. Understanding core collapse supernovae requires that we not only understand the explosion mechanism(s), but that we also understand all of their associated phenomenology, such as neutron star kicks(Fryer et al. 1998), their neutrino and gravitational wave and gamma-ray signatures, their bullet-like ejecta, and the polarization of their spectra (Wheeler 2000). To assemble this mosaic, knowing what mechanism is responsible for the explosion and for each observable, will require a systematic, coordinated study in multiple dimensions, at times focusing on certain critical aspects of the problem and at others integrating these aspects to achieve more sophisticated, richer models. This is the central goal of our collaboration.
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