  We revisit a singlet Majoron model in which neutrino masses arise from the
spontaneous violation of lepton number. If the Majoron obtains a mass of order
MeV, it can play the role of dark matter. We discuss constraints on the
couplings of the massive Majoron with masses of order MeV to neutrinos from
supernova data. In the dense supernova core, Majoron-emitting neutrino
annihilations are allowed and can change the signal of a supernova. Based on
the observation of SN1987A, we exclude a large range of couplings from the
luminosity and the deleptonization arguments, taking the effect of the
background medium into account. If the Majoron mass does not exceed the Q-value
of the experiment, the neutrino-Majoron couplings allow for neutrinoless double
beta decay with Majoron emission. We derive constraints on the couplings for a
Majoron mass of order MeV based on the phase space suppression and the
diminishing signal-to-background ratio due to the Majoron mass. The combination
of constraints from astrophysics and laboratory experiments excludes a large
range of neutrino-Majoron couplings in the mass range of intererest for Majoron
dark matter.
