  We consider a long-range force, mediated by an ultralight scalar, which can
give rise to violation of baryon number. This would lead to very different
lifetimes for nucleons in different astrophysical environments. Possible
signals of this scenario include a flux of O(10 MeV) solar neutrinos or
anomalous heating of old neutron stars; we find the latter to yield the
strongest current bounds, which could be improved in the coming years. The
ultralight scalar employed here can potentially be a good dark matter
candidate.
