Neutrino heating, convection, and the mechanism of Type-II supernova explosions.
Abstract
The role of neutrino heating and convective processes in the explosion mechanism of Type-II supernovae is investigated by one- and two-dimensional hydrodynamical simulations of the long-time evolution of the collapsed stellar core after the bounce at nuclear matter density and after the associated formation of the supernova shock. The parameters describing the neutrino emission from the collapsed stellar core are systematically varied. The possibility to obtain explosions turns out to be very sensitive to the physical conditions in and at the protoneutron star, in particular to its contraction and to the neutrino cooling inside of the gain radius. Yet, above a certain threshold for the core neutrino luminosity, stable and energetic explosions can be obtained in spherical symmetry, provided the energy deposition by neutrinos remains strong for a sufficiently long period. The explosion energy and time scale critically depend on the neutrino fluxes during the shock revival phase and on their temporal decay during the first few 100ms after shock formation. The threshold luminosity is a very sensitive function of the shock stagnation radius, because small radii of the stalled prompt shock lead to significantly higher neutrino loss from the hot and compact postshock layers, cause the region of neutrino heating to be very narrow, and reduce the heating time scale of the matter due to the high infall velocity. Repeating the simulations in two dimensions we find that strong convective processes occur in the collapsed stellar core in two spatially separate regions. One region of convection lies inside the neutrinosphere and another one is located in the neutrino-heated layer below the shock front. The convective mixing around the neutrinosphere is mainly driven by the negative lepton gradient, which is maintained by rapid loss of leptons from the semitransparent layers at the neutrinosphere. This considerably speeds up the deleptonization of the outer layers of the collapsed stellar core. Even 0.5 seconds after bounce a quasi-stationary pattern of convective motion is still present in the protoneutron star. Three-dimensional simulations reveal that rising and sinking convective elements have about half the size as in two dimensions with angular diameters between 10deg and 15deg, which causes corresponding anisotropies of the neutrino emission from the core. The large-scale convective overturn that takes place between the shock and the position of maximum neutrino heating is able to efficiently transport energy from the heated layer into the postshock region. This helps to stabilize the shock revival during the critical phase. It also leads to a faster increase and earlier saturation of the explosion energy, both because neutrino-heated material quickly moves out of the heating region and energy loss by the re-emission of neutrinos is reduced. The overturn pattern shows downflows of matter in narrow flux tubes and rising bubbles with typical angular extensions of 30deg to 60deg (in two dimensions). The material falling towards the neutron star loses lepton number, but readily absorbs energy in the neutrino-heated region, before it rises again. When the explosion gains momentum, this matter is not accreted onto the cooler, neutron-rich protoneutron star any more. After about 200-300ms the convective shell gets decoupled from the heating region and starts to move away from the neutron star to expand in an essentially self-similar way as a thick layer behind the outward propagating shock. The nearly spherically symmetrical "hot-bubble" region begins to develop and turbulence around the protoneutron star ceases. When the supernova shock passes the entropy step of the Si-O-interface about 400-500ms after bounce, the density inversion between the low-density hot-bubble region and the inhomogeneous shell steepens into a strong reverse shock that forms a sharp discontinuity in the neutrino wind from the nascent neutron star. The supernova shock is significantly deformed and velocity, density, temperature, and entropy in the postshock region exhibit large-scale variations with a contrast of up to a factor of 3. This must have consequences for the formation and spatial distribution of nucleosynthesis products and might help to explain the clumpiness, anisotropies, envelope and mantle mixing, and large velocities of iron group nuclei which were observed in the ejecta of SN 1987A and other supernovae. Although in this respect of essential importance, convection in the postshock region does not seem to be indispensable to get successful Type-II explosions via the neutrino-heating mechanism, nor do our simulations allow for the conclusion that its presence inevitably makes the mechanism successful and the explosions very energetic. As in spherical symmetry, the explosion energy and time scale are influenced by the contraction of the cooling neutron star and are very sensitive to the neutrino energy deposition and the size of the neutrino fluxes during a period of several hundred milliseconds after core bounce.
- Publication:
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Astronomy and Astrophysics
- Pub Date:
- February 1996
- Bibcode:
- 1996A&A...306..167J
- Keywords:
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- SUPERNOVAE: GENERAL;
- ELEMENTARY PARTICLES: NEUTRINOS;
- TURBULENCE;
- CONVECTION;
- SUPERNOVAE: INDIVIDUAL: SN 1987A