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 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 8 giants and 5 turnoff stars produces [ Fe/H ] = -0.17 \pm 0.07 ( sd ) and -0.07 \pm 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 ( §3.6 ) , Y ^ { 2 } isochrone comparisons lead to an age of 1.75 \pm 0.1 Gyr and an apparent modulus of ( m - M ) = 10.30 \pm 0.15 for the cluster , placing the center of the Li-dip at 1.35 \pm 0.03 M _ { \sun } .
Among the giants , five of 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 \pm 0.02 M _ { \sun } and noting the possible existence of 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 CMD ’ s of the intermediate-age clusters while implying a metallicity dependence for the central mass of the Li-dip given by M/M _ { \sun } = 1.38 \pm 0.04 + 0.4 \pm 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 .