THE HYADES LITHIUM PROBLEM REVISITED

Swenson, Stringfellow, & Faulkner (1990a) suggested that long-standing deficiencies in interior opacities could be responsible for an undere stimation of pre-main-sequence (PMS) lithium depletion in stellar mode ls. They showed that models with increased interior opacities could pr oduce excellent theoretical agreement with the pattern of depletion ob served in the Hyades G dwarfs. Preliminary opacities for appropriate c onditions then computed with the OPAL code were found to be consistent with the kind of opacity increments Swenson et al. had suggested. We now explore the self-consistent consequences of more complete opacity tables and several other physical improvements in both solar and Hyade s models. We examine their impact on PMS lithium depletion, the depth of the solar convective zone, and the deduced helium abundances for th e Sun and Hyades (the latter involving an appeal to the observed Hyade s mass-luminosity relationship). Adopted improvements are (1) OPAL int erior opacities (with solar photospheric or meteoritic iron abundances ), (2) new surface opacities (Alexander 1992), (3) calibrated paramete rs (from Proffitt & Michaud 1991a) for the pressure ionization prescri ption of the EFF (Eggleton, Faulkner, & Flannery 1973) equation of sta te, and (4) electrostatic corrections to the equation of state. (1) an d (3) significantly increase depletion while (2) significantly decreas es the depletion. While the ''best'' models deplete substantially more lithium than our early models, they do not deplete quite enough to fi t the observed pattern. (A notable ''peculiarity'' of the ''best'' sel f-consistent models so produced is that the deduced Hyades helium mass fraction falls short of the solar model value by approximately 0.02.) Finally, we explore a number of separate, additional possible changes that could produce an excellent self-consistent fit to both the Hyade s lithium depletion pattern and the observed mass-luminosity relations hip. Three such changes are: (i) an increase in opacity of approximate ly 14% in the 1-4 million degree range (which also produces a solar mo del with the correct convective depth as determined from oscillations) ; (ii) an increase in [Fe/H]Hyades from 0.13 to approximately 0.27 (wh ich also increases the deduced helium content of the Hyades to slightl y more than the solar value); (iii) a modest amount of convective over shooting, by approximately 0.08 of a pressure scale height, H(p). Sinc e some earlier, studies required overshooting by approximately 0.7H(p) to roughly fit the observed lithium depletion (while failing to fit i t in detail), our third alternative ''tune-up'' emphasizes how much of the older discrepancies, both quantitative and qualitative, have been removed by the opacities and physical improvements now incorporated i nto the ''best'' current models.