Neutrino helicity flip by Cerenkov emission and absorption of plasmons in supernova

We show that in a supernova core the longitudinal photon (plasmon) has a space-like dispersion and Cerenkov absorption and emission of such photons is kinematically allowed. If the neutrino has a non-zero magnetic moment, then helicity flipping Cerenkov absorption of a plasmon ${\nu }_L+\gamma \to {\nu }_R$ is the most efficient cooling mechanism of the supernova core, and this allows us to put a restrictive bound on the neutrino magnetic moment ${\mu }_{\nu }<0.7\times {10}^{-13}{\mu }_B$.