The Spite Lithium plateau: Ultrathin but postprimordial

We have studied 23 very metal-poor field turnoff stars, specifically chosen to enable a precise measurement 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] less than or simi lar to -2.5) to reduce the effects of systematic errors and have made parti cular efforts to minimize random errors. A typical statistical error for ou r abundances is 0.033 dex (1 sigma), which represents a factor of 2 improve ment on most previous studies. Our sample does not exhibit a trend with eff ective temperature, although the temperature range is limited. However, for -3.6 < [Fe/H] < -2.3 we do recover a dependence on metallicity at dA(Li)/d [Fe/H] = 0.118 +/- 0.023 (1 sigma) dex per dex, almost the same level as di scussed previously. Earlier claims for a lack of dependence of A(Li) on abu ndance are shown to have arisen probably from noisier estimates of effectiv e temperatures and metallicities, which have erased the real trend. The dep endence is concordant with theoretical predictions of Galactic chemical evo lution (GCE) of Li (even in such metal-poor stars) and with the published l evel of Li-6 in two of the stars of our sample, which we use to infer the G CE Li-7 contribution. One of the 23 stars, G186-26, was known already to be strongly Li-depleted. Of the remaining 22 objects, 21 have abundances cons istent with an observed spread about the metallicity trend of a mere 0.031 dex (1 sigma). Because the formal errors are 0.033 dex, we conclude that th e intrinsic spread is effectively zero at the very metal-poor halo turnoff. This is established at much higher precision than previous studies (simila r to 0.06-0.08 dex). The essentially zero intrinsic spread leads to the con clusion that either these stars have all changed their surface Li abundance s very uniformly, or else they exhibit close to the primordial abundance so ught for its cosmological significance. We cannot rule out a uniform deplet ion mechanism, but economy of hypothesis supports the latter interpretation . The lack of spread in the A(Li) abundances limits permissible depletion b y rotationally induced mixing models to less than 0.1 dex. Correcting for t he GCE contribution to both Li-6 and Li-7, we infer a primordial abundance A(Li)(p) similar or equal to 2.00 dex, with three systematic uncertainties of up to 0.1 dex each depending on uncertainties in the effective temperatu re scale, stellar atmosphere models, and correction for GCE. (This value re sts on an effective-temperature zero-point set by Magain's and Bell & Oke's b-y calibrations of metal-poor stars and the model atmospheres without con vective overshoot.) We predict that observations of Li in extremely low-met allicity stars, having [Fe/H] < - 3, will yield smaller A(Li) values than t he bulk of stars in this sample, consistent with a low primordial abundance . The difference between our field star observations and published M92 data suggests real field-to-cluster differences. This may indicate different an gular momentum evolutionary histories, with interactions between protostell ar disks in the dense globular cluster environments possibly being responsi ble. Further study of Li in globular clusters and in very metal-poor field samples is required to clarify the situation.