Core collapse supernovae (SN) are the final stages of stellar evolution in massive stars during which the central region collapses, forms a neutron star (NS) or black hole, and the outer layers are ejected. Recent explosion scenarios assume that the ejection is due to energy deposition by neutrinos into the envelope, but current models with detailed neutrino transport do not produce powerful explosions. There is new and mounting evidence for an asphericity and, in particular, for axial symmetry in several supernovae which may be hard to reconcile within the spherical picture. This evidence includes the observed high polarization and its variation with time, pulsar kicks, high velocity iron-group and intermediate-mass elements observed in remnants, and direct observations of the debris of SN 1987A. Some of the new evidence is discussed in more detail. To be in agreement with the observations, any successful mechanism must invoke some sort of axial symmetry for the explosion. Based on models in literature, we expect no such asymmetries from neutrino driven explosions.
As a limiting case for aspherical explosions, we consider jet-induced/dominated explosions of “classical” core collapse supernovae. Bipolar outflows may be formed as a consequence of an accretion disk around the central object which is formed just after the core collapse, MHD mechanisms, or, maybe, some new instabilities within the neutrino picture. Our study is based on detailed 3-D hydrodynamical and radiation transport models. We demonstrate the influence of the jet properties and of the underlying progenitor structure on the final density and chemical structure. Our calculations show that low velocity, massive jets can explain the observations. Both asymmetric ionization and density/chemical distributions have been identified as crucial for the formation of asymmetric photospheres. Even within the picture of jet-induced explosion, the latter effect alone fails to explain early polarization in core collapse supernovae with a massive, hydrogen-rich envelope such as SN 1999em. The need for an asymmetric distribution of freshly formed 56Ni may lend additional support for the idea that the explosion mechanism itself is asymmetric. Solving neutrino transport is an important ‘component’ to solve the SN problem but, apparently, not the complete solution. A successful model has to include all the effects, i.e. the core bounce, neutrino transport, convective flows and, in addition, significant effects due to rotation and, maybe, magnetic fields. Finally, we discuss observational consequences and tests.