Stellar Mixing and the Primordial Lithium Abundance

Abstract

We compare the properties of recent samples of the lithium abundances in halo stars to one another and to the predictions of theoretical models including rotational mixing, and we examine the data for trends with metal abundance. We apply two statistical tests to the data: a Kolomorgorov-Smirnov (K-S) test sensitive to the behavior around the sample median, and Monte Carlo tests of the probability to draw the observed number of outliers from the theoretical distributions. We find from a K-S test that in the absence of any correction for chemical evolution, the Ryan, Norris, & Beers (RNB) sample is fully consistent with mild rotational mixing induced depletion and, therefore, with an initial lithium abundance higher than the observed value. Tests for outliers depend sensitively on the threshold for defining their presence, but we find a 10%-45% probability that the RNB sample is drawn from the rotationally mixed models with a 0.2 dex median depletion with lower probabilities corresponding to higher depletion factors. Including or excluding the one upper limit in the sample changes the absolute probabilities but does not affect the overall conclusions. When chemical evolution trends (Li/H vs. Fe/H) are included in our analysis we find that the dispersion in the RNB sample is not explained by chemical evolution; the inferred bounds on lithium depletion from rotational mixing are similar to those derived from models without chemical evolution. Finally, we explore the differences between the RNB sample and other halo star data sets. We find that differences in the equivalent width measurements are primarily responsible for different observational conclusions concerning the lithium dispersion in halo stars. The different data sets are all consistent with mild stellar depletion, but the systematic errors arising from different observational data sets are a major component of the error budget and need to be addressed. The implications for cosmology are discussed. We find that the standard big bang nucleosynthesis predicted lithium abundance that corresponds to the deuterium abundance inferred from observations of high-redshift, low-metallicity QSO absorbers requires halo star lithium depletion in an amount consistent with that from our models of rotational mixing but inconsistent with no depletion.