3.2. Lithium in subdwarfs

The big breakthrough in relating ⁷Li to cosmology came with the brilliant discovery by Spite & Spite (1982a, b) of Li in metal-deficient subdwarf stars of the Galactic halo (extreme Population II). Relative to the Sun and most nearby stars, these are deficient in carbon and heavier elements by factors between 10 and 3 × 10⁴ and they are the oldest stars that we know, and yet in the range of effective temperatures 5500 K to 6500 K (where the main sequence turns off), as long as the metal-deficiency factor exceeds 50, they have virtually constant lithium abundance -10.2 < log(Li/H) < - 9.7 only a factor of 10 below young Population I stars (Rebolo, Molaro & Beckman 1988; cf. fig. 2). Below 5500 K, the Li abundance goes down with diminishing effective temperature, analogously to what is found in Pop. I.

Since the subdwarfs consist of virtually pristine material, it is natural to follow Spite & Spite in assuming that their lithium is all primordial ⁷Li (it is mainly ⁷Li and not ⁶Li; Spite, Maillard & Spite 1984) and the only question that arises is whether it represents the full primordial abundance or whether after such a long time since the formation of these stars the photospheric abundance has been depleted. The fact that the abundance has a constant plateau over a wide range in effective temperature suggests that the amount of depletion should be small, and this is indeed what is predicted by standard stellar evolution calculations (Deliyannis, Demarque & Kawaler 1990) which include diffusion but no non-classical effects like meridional circulation some of which must have been operating in Pop. I stars such as the Sun. Thus significant depletion is not entirely ruled out (Vauclair 1988; Mathews, Alcock & Fuller 1990; Krauss & Romanelli 1990), but it could very well be absent and, if so, then subdwarf Li gives a very impressive fit to SBBN predictions (which preceded its discovery) and limits from both above and below 1).

Figure 2. Plot of stellar 7Li abundances after Rebolo, Molaro & Beckman (1988). solid curve shows the prediction of a simple model in which 7Li is the sum of a primordial component and an additional component proportional to iron. The broken curve assumes the additional component proportional to oxygen. The two Sun symbols refer to meteoritic (identified as proto-solar) and photospheric (depleted) values respectively.