  The observed cosmic abundances of light elements are most consistent with
each other, and with the predictions of big bang nucleosynthesis,if, contrary
to the usual assumption, galactic chemical evolution reduces $(D+\ ^3He)/H$
with time. Chemical evolution models which do this require that low mass stars
destroy $^3He$ in the envelope gas that they return to the interstellar medium.
A simple argument based on the rates of limiting nuclear reactions shows that
the same giant branch mixing process which appears to be needed to explain the
observed $^{12}C/ ^{13}C$ and $C/N$ ratios in 1-- 2$\msol$ stars would indeed
also probably destroy $^3He$ by a large factor in the bulk of the envelope
material. The conclusion is that Galactic $^3He/H$ estimates should not be
trusted for setting an upper limit on primordial $(D+ ^3He)/H$. This removes
the strongest lower bound on the cosmic baryon density from big bang
nucleosynthesis and the only argument for abundant baryonic dark matter.

Astrophysics