  The existence of various anomalous stars, such as the first stars in the
universe or stars produced by stellar mergers, has been recently proposed. 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 - 30Msun, 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 ~10Msun
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. Firstly, the neutronization burst is less remarkable or disappears
completely for more massive models because the density is lower at the bounce.
Secondly, 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.
Further more, 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.
