1,767 research outputs found
Rotation of Horizontal Branch Stars in Globular Clusters
The rotation of horizontal branch stars places important constraints on
angular momentum evolution in evolved stars and therefore rotational mixing on
the giant branch. Prompted by new observations of rotation rates of horizontal
branch stars, we calculate simple models for the angular momentum evolution of
a globular cluster giant star from the base of the giant branch to the star's
appearance on the horizontal branch. We include mass loss, and infer the
accompanied loss of angular momentum for each of four assumptions about the
internal angular momentum profile. These models are compared to observations of
horizontal branch rotation rates in M13. We find that rapid rotation on the
horizontal branch can be reconciled with slow solid body main sequence rotation
if giant branch stars have differential rotation in their convective envelopes
and a rapidly rotating core, which is then followed by a redistribution of
angular momentum on the horizontal branch. We discuss the physical reasons why
these very different properties relative to the solar case may exist in giants.
Rapid rotation in the core of the main sequence precursors of the rapidly
rotating horizontal branch star, or an angular momentum source on the giant
branch is required for all cases if the rotational velocity of turnoff stars is
less than 4 km s. We suggest that the observed range in rotation rates
on the horizontal branch is caused by internal angular momentum redistribution
which occurs on a timescale comparable to the evolution of the stars on the
horizontal branch. The apparent lack of rapid horizontal branch rotators hotter
than 12 000 K in M13 could be a consequence of gravitational settling, which
inhibits internal angular momentum transport. Alternative explanations and
observational tests are discussed.Comment: 32 pages, 7 figures, submitted to the Astrophysical Journa
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
Preliminary Evaluation of the Kepler Input Catalog Extinction Model Using Stellar Temperatures
The Kepler Input Catalog (KIC) provides reddening estimates for its stars,
based on the assumption of a simple exponential dusty screen. This project
focuses on evaluating and improving these reddening estimates for the KIC's
giant stars, for which extinction is a much more significant concern than for
the nearby dwarf stars. We aim to improve the calibration (and thus
consistency) amongst various photometric and spectroscopic temperatures of
stars in the Kepler field by removing systematics due to incorrect extinction
assumptions. The revised extinction estimates may then be used to derive
improved stellar and planetary properties. We plan to eventually use the large
number of KIC stars as probes into the structure and properties of the Galactic
ISM.Comment: Proc. of the workshop "Asteroseismology of stellar populations in the
Milky Way" (Sesto, 22-26 July 2013), Astrophysics and Space Science
Proceedings, (eds. A. Miglio, L. Girardi, P. Eggenberger, J. Montalban
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