Li Evolution and the Open Cluster NGC 6819: A Correlation between Li Depletion and Spindown in Dwarfs More Massive Than the F-Dwarf Li-Dip *

Abstract

Spectroscopy of 333 NGC 6819 stars and Gaia astrometry are used to map Li evolution from the giant branch tip to 0.5 mag below the Li dip. Isochrone comparison with [Fe/H] = −0.04, based upon neural network spectroscopic analysis, produces an age of 2.25 (2.4) Gyr for E ( B − V ) = 0.16 (0.14) and ( m − M ) = 12.40 (12.29). Despite originating outside the Li dip, only 10% of single subgiants/giants have measurable Li. Above the Li dip, the limiting A (Li) for single stars is 3.2 ± 0.1 but the lower range is comparable to that found within the dip. The F-dwarf Li dip profile agrees with the Hyades/Praesepe, evolved forward. The Li level among stars populating the plateau fainter than the Li dip is A (Li) = 2.83 ± 0.16; the dispersion is larger than expected from spectroscopic error alone. Comparison of Li and V ROT distributions among turnoff stars in NGC 7789, NGC 2506, NGC 3680, and NGC 6819 indicates that rotational spindown from the main sequence is critical in defining the boundaries of the Li dip. For higher-mass dwarfs, spindown is likewise correlated with Li depletion, creating a second dip, but at higher mass and on a longer timescale. The Li distribution among evolved stars of NGC 6819 is more representative of the older M67, where subgiant and giant stars emerge from within the Li dip, than the younger NGC 7789, where a broad range in V ROT among the turnoff stars likely produces a range in mass among the giants.