Abstract
A new physical phenomenon is identified: volumetric stellar emission into gravitationally bound orbits of weakly coupled particles such as axions, moduli, hidden photons, and neutrinos. While only a tiny fraction of the instantaneous luminosity of a star (the vast majority of the emission is into relativistic modes), the continual injection of these particles into a small part of phase space causes them to accumulate over astrophysically long time scales, forming what I call a "stellar basin", in analogy with the geologic kind. The energy density of the Solar basin will surpass that of the relativistic Solar flux at Earth's location after only a million years, for any sufficiently long-lived particle produced through an emission process whose matrix elements are unsuppressed at low momentum. This observation has immediate and striking consequences for direct detection experiments---including new limits on axion parameter space independent of dark matter assumptions---and may also increase the prospects for indirect detection of weakly interacting particles around compact stars.
Highlights
I have identified and described a previously overlooked process in astroparticle physics. It is not just feeble interaction strengths, which allow for volumetric stellar emission, that make stars interesting sources of weakly coupled particles
A small but nonzero mass opens up the possibility of bound-orbit emission that fills up a stellar basin over time, a spectacular channel not available to massless photons
Preliminary estimates indicate that axions would quickly saturate to maximum occupation numbers [see Eq (17)] around isolated neutron stars from nuclear bremsstrahlung production, even with incredibly tiny couplings to nuclear matter
Summary
The energy loss rate for this bound emission component is typically very small [see Eq (6)] This peculiar ensemble of particles populates parts of phase space that may survive for millions to billions of years around isolated stars, even in the inner Solar System. I discuss general aspects of soft emission near a particle mass threshold, before delving into a case study of solar axions coupled to electrons. Based on these results, I present new limits on axion parameter space, and suggest the possible reinterpretation of the excess of Ref. The case study of dark photons is worked out in a companion paper [59]
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