2.3. Primordial D+ 3He
How do these results on D and 3He relate to primordial abundances? Since D is mainly destroyed by astration, its proto-solar abundance can be taken as a lower limit, but estimates of how much lower than primordial are rather model-dependent with respect to Galactic chemical evolution, depending on how much of the original Galaxy is still in the form of gas (10 to 20 per cent in the Solar neighbourhood), how much gas is returned to the ISM by each generation of stars (10 to 40 per cent) and whether there has been significant inflow of unprocessed gas from outside (cf. Pagel 1982). Thus factors from 2 to 10 are all quite conceivable. In the case of 3He, the situation is still more difficult because 3He is destroyed in astration through massive stars, but survives and can also be freshly produced in stars of lower mass (Dearborn, Schramm & Steigman 1986). Yang et al. (1984) pointed out that, because destruction of D leads to production of 3He, some of which survives further stellar processing, one can use existing abundances in the proto-Solar System to constrain the sum of primordial D and 3He by the equation
where y23p is the sum of primordial (D + 3He) / H and f is the fraction of 3He that survives astration through one generation of stars. In a somewhat more sophisticated treatment the second limit is increased to 10.9 × 10-5 (Olive et al. 1990), which is shown to a good approximation by a horizontal line in fig. 1. This gives what is currently the most stringent lower limit to in the framework of BBNS, shown by the left-most tall vertical line and the leftmost shorter double vertical line in fig. 1 for homogeneous and inhomogeneous models respectively. Equation (9) refers to closed Galactic chemical evolution models without inflow of unprocessed material. In models with inflow, which have some distinct advantages (Pagel 1989a), different arguments apply but the result is the same, since in such models it is virtually impossible to have less than 1/3 of primordial deuterium surviving (cf. Audouze & Tinsley 1974; Pagel 1982).
It would be nice to have an independent upper limit to primordial deuterium and there is a possibility that this will eventually be achieved from observations of absorption-line systems at high red-shifts having low metallicity, low Doppler broadening and large hydrogen column density, in front of quasars (Webb et al. 1991). The absence of any very definite results on deuterium in such clouds up to now suggests that the limit of 10-4 is not too likely to be violated.