Cerenkov radiation by neutrinos in a supernova core

Neutrinos with a magnetic dipole moment propagating in a medium with a velocity larger than the phase velocity of light emit photons by the Cerenkov process. The Cerenkov radiation is a helicity flip process via which a left-handed neutrino in a supernova core may change into a sterile right-handed one and free-stream out of the core. Assuming that the luminosity of such sterile right-handed neutrinos is less than $10^{53}$ ergs/sec gives an upper bound on the neutrino magnetic dipole moment $\mu_\nu < 0.2 \times 10^{-13} \mu_B$. This is two orders of magnitude more stringent than the previously established bounds on $\mu_\nu$ from considerations of supernova cooling rate by right-handed neutrinos.