On nonprimordial deuterium production by accelerated particles

Abstract

Deuterium plays a crucial role in cosmology because the primordial D/H abundance, in the context of big bang nucleosynthesis (BBN) theory, yields a precise measure of the cosmic baryon content. Observations of D/H can limit or measure the true primordial abundance because D is thought to be destroyed by stars and thus D/H monotonically decreases after BBN. Recently, however, Mullan & Linsky have pointed out that D arises as a secondary product of neutrons in stellar flares that then capture on protons via n + p → d + γ and that this could dominate over direct D production in flares. Mullan & Linsky note that if this process is sufficiently vigorous in flaring dwarf stars, it could lead to significant non-BBN D production. We have considered the production of D in stellar flares, both directly and by n capture. We find that for reasonable flare spectra, n/d ≲ 10 and (n + d)/⁶Li ≲ 400, both of which indicate that the n-capture channel does not allow for Galactic D production at a level that will reverse the monotonic decline of D. We also calculate the 2.22 MeV gamma-ray line production associated with n capture and find that existing COMPTEL limits also rule out significant D production in the Galaxy today. Thus, we find that flares in particular and neutron captures in general are not an important Galactic source of D. On the other hand, we cannot exclude that flare production might contribute to recent Far Ultraviolet Spectroscopic Explorer (FUSE) observations of large variations in the local interstellar D/H abundance; we do, however, offer important constraints on this possibility. Finally, since flare stars should inevitably produce some n-capture events, a search for diffuse 2.22 MeV gamma rays by INTEGRAL can further constrain (or measure!) Galactic deuterium production via n-capture.