AAS 201st Meeting, January, 2003
Session 124. WIYN Open Cluster Study
Poster, Thursday, January 9, 2003, 9:20am-4:00pm, Exhibit Hall AB

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[124.03] WIYN Open Cluster Study: A Solution to the Pleiades High-Lithium-In-Rapid-Rotators Problem? Clues from NGC 2451A and B.

S. J. Margheim, C. P. Deliyannis (Indiana Univ.), J. R. King (UNLV), A. Steinhauer (Indiana Univ.)

The traditional methods of Lithium abundance determination through curve-of-growth (COG) analysis has led to the result of an enormous (nearly 2dex) spread of Li abundances in the cool stars (5400K > Teff > 4500K) of the Pleiades open cluster. The abundance of Li in these stars is roughly correlated with rotation (more rapid rotators have a greater Li abundance). Attempts that have been made to explain this dispersion as either a real dispersion due to rotation's effect on stellar structure and thus Li depletion, or as an apparent dispersion due to chromospheric effects on the abundance analysis, have failed.

NGC 2451 A and B are co-eval, young (~100 Myr), metal-rich, open clusters with many fast rotators. Our initial COG analysis revealed a Li dispersion similar to that seen the Pleiades, but extending to even higher temperatures. However, when a) sufficiently high S/N data are employed, b) cluster metallicity and other stellar parameters are carefully taken into account, and c) detailed spectral synthesis is applied, we find that most of the Li dispersion goes away. The rapid rotators' are now barely above the mean A(Li)-Teff trend. The main factor is that the excess Li abundance in fast rotators is due to blending from neighboring lines and is not properly accounted for in COG analyses. This effect is clearly demonstrated by emprically spinning up a slow rotator's spectrum: the spun-up A(Li) closely matches the A(Li) of real rapid rotators of similar Teff. This blending effect also explains why a COG-based Li dispersion is seen in hotter stars of NGC 2451 (as compared to the Pleiades) since NGC 2451 is 0.15-0.2dex more metal-rich than the Pleiades.

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