Misc,
Surface rotation of Kepler red giant stars
(Jul 19, 2017)
Abstract
Kepler allows the measurement of starspot variability in a large sample of
field red giants for the first time. With a new method that combines
autocorrelation and wavelet decomposition, we measure 361 rotation periods from
the full set of 17,377 oscillating red giants in our sample. This represents
2.08\% of the stars, consistent with the fraction of spectroscopically detected
rapidly rotating giants in the field. The remaining stars do not show enough
variability to allow us to measure a reliable surface rotation period. Because
the stars with detected rotation periods have measured oscillations, we can
infer their global properties, e.g. mass and radius, and quantitatively
evaluate the predictions of standard stellar evolution models as a function of
mass. Consistent with results for cluster giants, when we consider only the
4881 intermediate-mass stars, M>2.0 M\_Sun from our full red giant sample, we do
not find the enhanced rates of rapid rotation one would expect from angular
momentum conservation. We therefore suggest that either enhanced angular
momentum loss or radial differential rotation must be occurring in these stars.
Finally, when we examine the 575 low-mass (M<1.1 M\_Sun) red clump stars in our
sample, which we would have expected to exhibit slow (non-detectable) rotation,
15\% of them actually have detectable rotation. This suggests a high rate of
interactions and stellar mergers on the red giant branch.
