121 research outputs found
A Tale of Two Anomalies: Depletion, Dispersion, and the Connection Between the Stellar Lithium Spread and Inflated Radii on the Pre-Main Sequence
We investigate lithium depletion in standard stellar models (SSMs) and main
sequence (MS) open clusters, and explore the origin of the Li dispersion in
young, cool stars of equal mass, age and composition. We first demonstrate that
SSMs accurately predict the Li abundances of solar analogs at the zero-age main
sequence (ZAMS) within theoretical uncertainties. We then measure the rate of
MS Li depletion by removing the [Fe/H]-dependent ZAMS Li pattern from three
well-studied clusters, and comparing the detrended data. MS depletion is found
to be mass dependent, in the sense of more depletion at low mass. A dispersion
in Li abundance at fixed is nearly universal, and sets in by
200 Myr. We discuss mass and age dispersion trends, and the pattern is
mixed. We argue that metallicity impacts the ZAMS Li pattern, in agreement with
theoretical expectations but contrary to the findings of some previous studies,
and suggest Li as a test of cluster metallicity. Finally, we argue that a
radius dispersion in stars of fixed mass and age, during the epoch of pre-MS Li
destruction, is responsible for the spread in Li abundances and the correlation
between rotation and Li in young cool stars, most well known in the Pleiades.
We calculate stellar models, inflated to match observed radius anomalies in
magnetically active systems, and the resulting range of Li abundances
reproduces the observed patterns of young clusters. We discuss ramifications
for pre-MS evolutionary tracks and age measurements of young clusters, and
suggest an observational test.Comment: 29 pages, 20 Figures, 4 Tables. A short video discussing the key
results can be found at this link: http://youtu.be/8576JQ0WkY
Rotation, inflation, and lithium in the Pleiades
The rapidly rotating cool dwarfs of the Pleiades are rich in lithium relative
to their slowly rotating counterparts. Motivated by observations of inflated
radii in young, active stars, and by calculations showing that radius inflation
inhibits pre-main sequence (pre-MS) Li destruction, we test whether this
pattern could arise from a connection between stellar rotation rate and radius
inflation on the pre-MS. We demonstrate that pre-MS radius inflation can
efficiently suppress lithium destruction by rotationally induced mixing in
evolutionary models, and that the net effect of inflation and rotational mixing
is a pattern where rotation correlates with lithium abundance for , and anti-correlates with lithium abundance for , similar to the empirical trend in the Pleiades. Next, we adopt
different prescriptions for the dependence of inflation on rotation, and
compare their predictions to the Pleiades lithium/rotation pattern. We find
that if a connection between rotation and radius inflation exists, then the
important qualitative features of this pattern naturally and generically emerge
in our models. This is the first consistent physical model to date that
explains the Li--rotation correlation in the Pleiades. We discuss plausible
mechanisms for inducing this correlation and suggest an observational test
using granulation.Comment: 17 pages, 9 figures. A short video summarizing the results of our
paper can be found here: https://www.youtube.com/watch?v=pnXFBCRQgd
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