Name/Affiliation: Garrett Somers (The Ohio State University Astronomy Department)
Born Different: How Rapid Rotation on the Pre-Main Sequence Inhibits Lithium Destruction in Cool Stars
We explore the origins of the zero-age main sequence (ZAMS) open cluster lithium pattern, including the Li dispersion in young, cool stars of equal mass, age and composition. We first demonstrate that standard stellar models (SSMs) accurately predict the Li abundance of solar analogs at the ZAMS within theoretical uncertainties. We then argue that a radius dispersion in stars of equal mass, during the epoch of pre-MS Li destruction, is responsible for the spread in Li abundances in cooler stars, most well known in the Pleiades. Inflated radii have been observed in detached eclipsing binaries and in single star interferometric measurements, and are seen to increase with faster rotation. The base of the convection zone is cooled when a star is inflated, greatly suppressing the rate of Li depletion during the pre-MS. This causes suppressed Li destruction in rapid rotators, resulting in a dispersion at fixed mass, and a correlation between Li and rotation at the ZAMS. We calculate stellar models, inflated to match the upper envelope of observed radius anomalies, and the resulting range of Li abundances accurately reproduces the empirical patterns of several young clusters. We discuss ramifications for pre-MS evolutionary tracks and age measurements of young clusters, and suggest an observational test. Furthermore, we use our validated SSMs to measure the rate of Li destruction during the MS, by subtracting the [Fe/H]-dependent ZAMS Li pattern from several MS open clusters and comparing the resulting data. With this method, we identify strong mass trends in the rate of MS Li depletion, and study the emergence of dispersion in Li abundances at fixed effective temperature, which is found to be a general feature of open clusters.