  Sterile neutrinos of keV masses are one of the most promising candidates for
the warm dark matter, which could solve the small-scale problems encountered in
the scenario of cold dark matter. We present a detailed study of the production
of such sterile neutrinos in a supernova core, and derive stringent bounds on
the active-sterile neutrino mixing angles and sterile neutrino masses based on
the standard energy-loss argument.
