LITHIUM IN THE HYADES .1. NEW OBSERVATIONS

Sixty-eight main-sequence members of the Hyades cluster with spectral types ranging from F7 to K1 V have been observed at Li I lambda6707 to determine whether solar-like stars of a given mass, age, and initial composition have a unique surface lithium abundance Li/H. The data are compared with previous Li I observations in the Hyades, and similarit ies and differences are discussed. For the sample as a whole, the obse rved distribution of lithium abundances is inconsistent with a unique relationship between lithium abundance and mass. Furthermore, there is strong observational evidence for main-sequence lithium depletion whe n the data are compared with observations in younger clusters, even fo r masses that are not predicted to experience such depletion in standa rd stellar models. Physics not included in standard stellar models is therefore required to explain the lithium abundances of low-mass stars . The degree of dispersion in the Li abundances is a function of mass. The observed dispersion is relatively large in the late F stars (5900 -6200 K) and generally small in the G and early K stars. A statistical analysis indicates that an intrinsic dispersion in the lithium abunda nces of the G stars (5300-5900 K) is present at the 8 sigma confidence level. However, a typical randon B-V error exceeding 0.009m would cau se the derived precision to decline below 2 sigma. A second, high-prec ision (+/- 10 K) relative temperature scale would be desirable for con firming the existence of a lithium abundance dispersion in this temper ature range. Among the G stars, vB 9 appears to be highly overdepleted with respect to the trend. Observational errors are substantially lar ger for the cooler stars, making an analysis of dispersion below 5300 K uncertain. We report a number of detections and upper limits among t he coolest stars; the detections may represent the older counterparts to the Li-rich late-type stars seen in the Pleiades. We have discovere d two new, short period binaries with lithium abundances well above th e mean trend for other stars: BD + 22-degrees 669 and BD + 23-degrees 635. These tidally locked binaries contain the coolest stars in our pr ogram, and we report Li i detections for both. We have also confirmed the high lithium abundances previously found for two other tidally loc ked systems: vB 22 and 62. These results suggest that a star's rotatio nal history affects its surface lithium abundance.