2020ApJ...888...28B

The surface content of lithium (Li) and beryllium (Be) provides insight into the mixing and circulation mechanisms in stellar interiors. The old open cluster, M67, has been well-studied for Li abundances in both main-sequence and evolved stars. The Be abundances give us a probe to a deeper level in stars. We have taken high-resolution spectra with Keck I with HIRES to determine Be abundances along the subgiant branch of M67, where there are dramatic depletions of Li. These subgiants range in mass from 1.26 to 1.32 M_☉ and have evolved from main-sequence stars that would have occupied the region of the Li-Be dip found in younger clusters. Lithium abundances have been adjusted to the same scale for 103 stars in M67 by Pace et al. The more massive stars-now the coolest and furthest-evolved from the main sequence-show a drop in Li by a factor of 400 across the subgiant branch. Our new Be abundances also show a decline, but by a factor of ∼50. The two elements decline together with Li showing a steeper decline in these subgiants than it does in the Li-Be dip stars. The relative decline in Be abundance compared to Li is remarkably well fit by the models of Sills & Deliyannis, made specifically for the subgiants in M67. Those models include the effects of mixing induced by stellar rotation. These M67 subgiants show the effects of both main-sequence depletion and post-main-sequence dilution of both Li and Be.