Effect of anisotropic neutrino radiation on supernova explosion energy

Since SN 1987A, many observations have indicated that supernova explosions are not spherical. The cause of the asymmetric explosion is still controversial (e.g., asymmetry in the envelope, the convective engine in the central core or in the proto-neutron star). In our previous study, anisotropic neutrino radiation has been proposed as an explanation for this asymmetry. In this paper we carried out a series of systematic multidimensional numerical simulations in order to investigate the effect of anisotropic neutrino radiation itself on the supernova explosion energy. The neutrino luminosity and the degree of anisotropy in neutrino radiation were assumed as input parameters, and the numerical results for various parameters were compared with each other. It was found that only a few percent of anisotropy in the neutrino emission distribution is sufficient to increase the explosion energy by a large factor. The explosion energy calculated so far in many supernova models has tended to be too short to explain the observation. Anisotropy of 10% in neutrino radiation roughly corresponds to an enhancement of 4% in total neutrino luminosity as far as the explosion energy is concerned. The increase in the explosion energy due to anisotropic neutrino radiation can be explained as follows. Anisotropically emitted neutrinos locally heat the supernova matter and revive a stalled shock wave in the direction of enhanced radiation. The expansion of the gas by the shock propagation results in a decrease in the neutrino cooling (emission) rate that rapidly decreases with the matter temperature. It is this suppression of energy loss that contributes largely to the increase in explosion energy. The efficiency of neutrino heating (absorption) itself is almost unchanged between anisotropic and spherical models with available energy fixed for neutrinos. In order for a stalled shock wave to revive, enhancement of the local intensity in the neutrino flux is of great importance, rather than that of the total neutrino luminosity over all the solid angle. It is first pointed out that such local neutrino heating is capable of triggering a supernova explosion. Anisotropic neutrino radiation is considered to be a plausible mechanism for a "successful" explosion other than the so far suggested "convective trigger."