Lithium in the intermediate-age open cluster, NGC 3680

Abstract

High-dispersion spectra centered on the Li 6708 Å line have been obtained for 70 potential members of the intermediate-age open cluster NGC 3680, with an emphasis on stars in the turnoff region of the cluster color-magnitude diagram (CMD). A measurable Li abundance has been derived for 53 stars, 39 of which have radial velocities and proper motions consistent with cluster membership. After being transferred to common temperature and abundance scales, previous Li estimates have been combined to generate a sample of 49 members, 40 of which bracket the cluster Li-dip. Spectroscopic elemental analysis of eight giants and five turnoff stars produces [Fe/H] = -0.17 ± 0.07 (sd) and -0.07 ± 0.02 (sd), respectively. We also report measurements of Ca, Si, and Ni which are consistent with scaled-solar ratios within the errors. Adopting [Fe/H] = -0.08 (Section 3.6), Y2 isochrone comparisons lead to an age of 1.75 ± 0.1 Gyr and an apparent modulus of (m - M) = 10.30 ± 0.15 for the cluster, placing the center of the Li-dip at 1.35 ± 0.03 M⊙. Among the giants, five of the nine cluster members are now known to have measurable Li with A(Li) near 1.0. A combined sample of dwarfs in the Hyades and Praesepe is used to delineate the Li-dip profile at 0.7 Gyr and [Fe/H] = +0.15, establishing its center at 1.42 ± 0.02 M ± and noting the possible existence of a secondary dip on its red boundary. When evolved to the typical age of the clusters NGC 752 (age = 1.45 Gyr, (m - M) = 8.4), IC 4651 (age = 1.5 Gyr, (m - M) = 10.4), and NGC 3680, the Hyades/Praesepe Li-dip profile reproduces the observed morphology of the combined Li-dip within the CMDs of the intermediate-age clusters while implying a metallicity dependence for the central mass of the Li-dip given by M/M ± = 1.38 ± 0.04 + 0.4 ± 0.2 [Fe/H]. The implications of the similarity of the Li-dichotomy among giants in NGC 752 and IC 4651 and the disagreement with the pattern among NGC 3680 giants are discussed. © 2009. The American Astronomical Society. All rights reserved.