New stellar constraints on dark photons

We consider the stellar production of vector states V within the minimal model of "dark photons". We show that when the Stückelberg mass of the dark vector becomes smaller than plasma frequency, the emission rate is dominated by the production of the longitudinal modes of V, and scales as κ2mV2, where κ and m_V are the mixing angle with the photon and the mass of the dark state. This is in contrast with widespread assertions in the literature that the emission rate decouples as the forth power of the mass. We derive ensuing constraints on the (κ, m_V) parameter space by calculating the cooling rates for the Sun and horizontal branch stars. We find that stellar bounds for m_V<10eV are significantly strengthened, to the extent that all current "light-shining-through-wall" experiments find themselves within deeply excluded regions.