  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 which then capture on protons via n+p \to d + gamma, 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
which 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 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 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.
