Membership, metallicity and lithium abundances for solar-type stars in NGC 6633

We present spectroscopic observations of candidate F-, G- and K-type stars in NGC 6633, an open cluster with a similar age to the Hyades. From the radial velocities and metal-line equivalent widths we identify 10 new cluster members including one short-period binary system. Combining this survey with that of Jeffries, we identify a total of 30 solar-type members. We have used the F and early G stars to estimate [Fe/H] = 0/096 +/- 0.081 spectroscopically for NGC 6633. When compared with iron abundances in other clusters, determined in a strictly comparable way, we can say with more precision that NGC 6633 has 0.074 +/- 0.041 dex less iron than the Pleiades and 0.206 +/- 0.040 dex less iron than the Hyades. A photometric estimate of the overall metallicity from the locus of cluster members in the B-V, V-I-c plane yields [M/H] = 0.04 +/- 0.10. A new estimate, based upon isochrones that are empirically tuned to fit the Pleiades, gives a distance modulus to NGC 6633 that is 2.41 +/- 0.09 larger than that of the Pleiades. Lithium abundances have been estimated for the NGC 6633 members and compared with consistently determined Li abundances in other clusters. Several mid-F stars in NGC 6633 show strong Li depletion at approximately the same effective temperature as that at which this phenomenon is seen in the Hyades. At lower temperatures the Li abundance patterns in several open clusters with similar ages (NGC 6633, Hyades, Praesepe and Coma Berenices) are remarkably similar, despite their differing [Fe H]. There is, however, evidence that the late G and K stars of NGC 6633 have depleted less Li than their Hyades counterparts. This qualitatively agrees with models for pre-main-sequence (PMS) Li depletion that feature only convective mixing, but these models cannot simultaneously explain why these stars have in turn depleted Li by more than 1 dex compared with their zero-age main-sequence counterparts in the Pleiades. Two explanations are put forward. The first is that elemental abundance ratios, particularly [O/Fe], may have non-solar values in NGC 6633 and would have to be higher than in either the Hyades or the Pleiades. The second is that additional non-convective mixing, driven by angular momentum loss, causes additional photospheric Li depletion during the first few hundred Myr of main-sequence evolution.