THE SPITE LITHIUM PLATEAU. ULTRATHIN BUT POSTPRIMORDIAL

摘要

We have studied 23 very metal-poor field turnoff stars, specifically chosen to enable a precise measure- ment of the dispersion in the lithium abundance of the Spite Li plateau. We concentrated on stars having a narrow range of effective temperature and very low metallicities ([Fe/H] approx< -2.5) to reduce the effects of systematic errors and have made particular efforts to minimize random errors. A typical sta- tistical error for our abundances is 0.033 dex (1 #sigma#), which represents a factor of 2 improvement on most previous studies. Our sample does not exhibit a trend with effective temperature, although the tem- perature range is limited. However, for -3.6 < [Fe/H] < -2.3 we do recover a dependence on metal- licity at dA(Li)/d[Fe/H] = 0.1 18 +- 0.023 (l #sigm#) dex per dex, almost the same level as discussed previously. Earlier claims for a lack of dependence of A(Li) on abundance are shown to have arisen probably from noisier estimates of effective temperatures and metallicities, which have erased the real trend. The depen- dence is concordant with theoretical predictions of Galactic chemical evolution (GCE) of Li (even in such metal-poor stars) and with the published level of ~6Li in two of the stars of our sample, which we use to infer the GCE ~7Li contribution. One of the 23 stars, G186-26, was known already to be strongly Li-depleted. Of the remaining 22 objects, 21 have abundances consistent with an observed spread about the metallicity trend of a mere 0.03l dex (1 #sigma#a). Because the formal errors are 0.033 dex, we conclude that the intrinsic spread is eff