The Hyades Lithium Problem Revisited
Abstract
We now explore the self-consistent consequences of more complete opacity tables and several other physical improvements in both solar and Hyades models. We examine their impact on pre-main-sequence (PMS) lithium depletion, the depth of the solar convective zone, and the deduced helium abundances for the Sun and Hyades (the latter involving an appeal to the observed Hyades mass-luminosity relationship). Adopted improvements are (1) OPAL interior opacities (with solar photospheric or meteoritic iron abundances), (2) new surfaces opacities (Alexander 1992), (3) calibrated parameters (from Proffitt & Michaud 1991a) for the pressure ionization prescription of the EFF (Eggleton, Faulkner, & Flannery 1973) equation of state, and (4) electrostatic corrections to the equation of state. (1) and (3) significantly increase depletion while (2) significantly decreases the depletion. While the 'best' models deplete substantially more lithium than our early models, they do not deplete quite enough to fit the observed pattern. (A notable 'peculiarity' of the 'best' self-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 Hyades lithium depletion pattern and the observed mass-luminosity relationship. Three such changes are: (1) an increase in opacity of approximately 14% in the 1-4 million degree range (which also produces a solar model with the correct convective depth as determined from oscillations); (2) an increase in (Fe/H)Hyades from 0.13 to approximately 0.27 (which also increases the deduced helium content of the Hyades to slightly more than the solar value); (3) a modest amount of convective overshooting, by approximately 0.08 of a pressure scale height, Hp. Since some earlier, studies required overshooting by approximately 0.7Hp to roughly fit the observed lithium depletion (while failing to fit it 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 into the 'best' current models.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- April 1994
- DOI:
- 10.1086/173985
- Bibcode:
- 1994ApJ...425..286S
- Keywords:
-
- Computational Astrophysics;
- Depletion;
- Dwarf Stars;
- Equations Of State;
- G Stars;
- Lithium;
- Open Clusters;
- Pre-Main Sequence Stars;
- Computer Programs;
- Opacity;
- Solar Convection (Astronomy);
- Stellar Luminosity;
- Stellar Mass;
- Stellar Models;
- Astrophysics