Numerical study of stellar core collapse and neutrino emission:: Probing the spherically symmetric black hole progenitors with 3-30 M⊙ iron cores

The existence of various anomalous stars, such as the first stars in the universe or stars produced by stellar mergers, has been proposed recently. Some of these stars will result in black hole formation. In this study we investigate iron-core collapse and black hole formation systematically for the iron-core mass range of 3-30 M-circle dot, which has not been studied well so far. Models used here are mostly isentropic iron cores that may be produced in merged stars in the present universe, but we also employ a model that is meant for a Population III star and is obtained by evolutionary calculation. We solve numerically the general relativistic hydrodynamics and neutrino transfer equations simultaneously, treating neutrino reactions in detail under spherical symmetry. As a result, we find that massive iron cores with similar to 10 M-circle dot unexpectedly produce a bounce, owing to the thermal pressure of nucleons before black hole formation. The features of neutrino signals emitted from such massive iron cores differ in time evolution and spectrum from those of ordinary supernovae. First, the neutronization burst is less remarkable or disappears completely for more massive models, because the density is lower at the bounce. Second, the spectra of neutrinos, except the electron type, are softer, owing to the electron-positron pair creation before the bounce. We also study the effects of the initial density profile, finding that the larger the initial density gradient is, the more steeply the neutronization burst declines. Furthermore, we suggest a way to probe into the black hole progenitors from the neutrino emission and estimate the event number for the currently operating neutrino detectors.