Membership and lithium abundances of late-type stars in NGC 6633

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Abstract
NGC 6633 is a young, open cluster with a similar age to the Hyades and Praesepe, but lower metallicity. Using a new photometric survey and proper-motion information, a sample of candidate members of NGC 6633 have been observed spectroscopically. Radial velocities are used to confirm membership for 21 late-type stars including two spectroscopic binaries. By combining the spectroscopy with existing UBV photometry, it is shown that NGC 6633 has a significantly lower metallicity than the Hyades, but not much lower than [M/H] = 0.0. The projected equatorial velocities of the cluster members are compared with Hyades rotation data and found to be consistent. The lithium abundances, determined from the Li i 6708-Å line, are indistinguishable from those in the Hyades for F stars, but show a different pattern of depletion in the cooler stars that can be qualitatively attributed to the lower metallicity. Three early K stars have log N(Li) >1.2, whereas only smaller upper limits can be found in the Hyades and Praesepe. Comparison with younger Pleiades cool stars, which have a similar metallicity to NGC 6633, shows that Li depletion apparently occurs in G and K stars while they are on the main sequence. Standard models, incorporating only convective mixing, underpredict the depletion in both the Hyades and NGC 6633, but match the lower envelope of the Pleiades distribution. These data argue strongly against enhanced interior opacities as a way of increasing Li depletion in the standard models, and instead point to extra mixing mechanisms on the main sequence combined with a rotation-dependent Li-depletion inhibition process acting during pre-main-sequence evolution.

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