OXYGEN ABUNDANCES IN F-TYPE STARS OF THE HYADES AND THE URSA MAJOR GROUP

We have derived the oxygen abundances of 50 F-type main-sequence stars , belonging to the Hyades open cluster, the Ursa Major group, and the field, using an NLTE analysis of the infrared triplet lines of O I at lambdalambda7771-7775. The stars of the middle F-range (6500 < T(eff) < 6900 K) have previously been shown to exhibit a marked reduction in lithium abundance compared with both cooler and hotter objects in the same systems: the ''Li gap,'' for which several explanations have been proposed. Our results here show a substantial measure of uniformity i n the oxygen abundances over the range in T(eff) between 5800 and 7400 K. The data might indicate a small dip (<0.1 dex) in the oxygen abund ance for stars located in the Li gap. Microscopic diffusion seems to b e the only known mechanism able to produce an oxygen dip. If this mech anism were also responsible for the Li gap, it would have to account f or a depletion of up to two orders of magnitude in lithium and, at the same time, less than 0.1 dex in oxygen. We note, however, that turbul ent mixing could modify a microscopic diffusion pattern in order to yi eld the observed abundances. Our oxygen abundances for the Hyades and the Ursa Major group stars fit well a trend of increasing [O/H] ratio with increasing metallicity, as observed in the Galactic disk for star s in the range -0.8 less-than-or-equal-to [Fe/H] less-than-or-equal-to +0.3. There is, however, a strong scatter in this trend for stars wit h [Fe/H] > 0. On the other hand, the [O/H] cluster values imply an evo lution of [O/H] with age which is flatter than that inferred for field stars only.