Supernova bound on keV-mass sterile neutrinos reexamined

Active-sterile neutrino mixing is strongly constrained for mₛ100 keV to avoid excessive energy losses from supernova cores. For smaller mₛ, matter effects suppress the effective mixing angle except for a resonant range of energies where it is enhanced. We study the case of ν_τ-νₛ mixing where a ν_τ-ν_τ asymmetry builds up due to the strong excess of νₛ over νₛ emission or vice versa, reducing the overall emission rate. In the warm dark matter range mₛ10 keV the mixing angle is essentially unconstrained.