Primordial Lithium from Globular Cluster Turn-Off Stars: M13 AND M71

TL;DR

This study measures lithium abundances in unevolved stars of globular clusters M13 and M71 to compare with Big Bang nucleosynthesis predictions, revealing a consistent maximum Li level despite cluster age and metallicity differences.

Contribution

It provides new high-resolution spectroscopic measurements of Li in globular cluster turn-off stars, highlighting the spread due to stellar rotation and mixing, and compares these with BBN predictions.

Findings

Maximum Li abundance is consistent across clusters despite age and metallicity differences.

Observed Li abundance is lower than BBN predictions by a factor of about 1.7.

Li spread attributed to stellar rotation and mixing effects.

Abstract

During Big Bang nucleosynthesis (BBN) in the first 15 minutes of the Universe some $^7$Li was created along with isotopes of H and He. The determination of that primordial value of Li can help constrain the conditions at that time. The oldest stars with known ages can be found in globular clusters which have well-determined ages through stellar evolution models. High-resolution spectra have been obtained with the Keck I telescope and HIRES of Li in several unevolved stars in the clusters M13 and M71 with V magnitudes of 17.6 -- 17.9. Abundances of Li have been determined with spectrum synthesis techniques and show a range of a factor of 4. We attribute that spread to differences in initial angular momentum resulting in different amounts of spin-down, related mixing, and destruction of Li. Our results are compared with similar results for main-sequences and turn-off stars in other globular clusters. The range in age of these clusters is 11.2 to 14.2 Gyr for an age span of 3 Gyr. These clusters range in [Fe/H] from $-$0.75 to $-$2.24 corresponding to a factor of 30 in metallicity. The maximum in the Li abundance for these unevolved stars in all eight clusters is the same corresponding to Li/H = 3.16 x 10$^{-10}$ while the predicted Li abundance, based on the deuterium abundance and the BBN predictions, is 5.24 x 10$^{-10}$.