Deuterium holds a special place in cosmology and Big Bang nucleosynthesis (BBN) as an excellent cosmic “baryometer”. Its strong dependence on baryon density and the fact that it is destroyed in stars, and thus monotonically decreases from its primordial value, makes deuterium an important probe of the cosmic baryon content and the early universe physics. All astrophysical deuterium productions sites were considered and it was concluded that there is no significant production of deuterium in any of these, and that the Big Bang nucleosynthesis remains the only important source. Thus recent observations of large local variations of deuterium abundance are quite puzzling. As a solution to this problem, it was proposed that neutrons, produced in spallation reactions in stellar flares, undergo radiative capture onto protons with emission of 2.2 MeV gamma-ray line (n+p→d+γ2.2), and can thus yield a potentially significant amount of deuterium. Though this mechanism can at least partially explain ISM variations of deuterium, it would have had important consequences for cosmology, namely, it would have called in question the spectacular agreement between high-redshift deuterium measurements and the predictions of the BBN with the WMAP baryon density. We have shown in multiple ways that this deuterium production channel is not important compared to Galactic deuterium destruction rate. The constraints come from careful consideration of spallation reactions in flares as well as from COMPTEL limits to 2.2 MeV gamma-ray line.

Reference: Prodanović, T. & Fields, B. D. 2003, ApJ 597, 48 (get from astro-ph/)