Rotation of Kepler field dwarfs and sub giants: Spectroscopic $v \sin I$ from APOGEE

TL;DR

This study analyzes spectroscopic measurements of stellar rotation in Kepler field stars, revealing rotation trends, binary effects, and angular momentum evolution, with implications for stellar aging and binary identification.

Contribution

It provides a comprehensive analysis of stellar rotation using APOGEE spectroscopic data, highlighting new insights into binary effects and stellar evolution.

Findings

Detection threshold of 10 km/s for stellar rotation.

Clear distinction between blue stragglers and field turnoff stars.

Evidence for a transition in rotation on the subgiant branch.

Abstract

We use 5,337 spectroscopic $v \sin i$ measurements of Kepler dwarfs and subgiants from the APOGEE survey to study stellar rotation trends. We find a detection threshold of 10 km/s, which allows us to explore the spindown of intermediate-mass stars leaving the main sequence, merger products, young stars, and tidally-synchronized binaries. We see a clear distinction between blue stragglers and the field turnoff in $\alpha$-rich stars, with a sharp rapid rotation cutoff for blue stragglers consistent with the Kraft break. We also find rapid rotation and RV variability in a sample of red straggler stars, considerably cooler than the giant branch, lending credence to the hypothesis that these are active, tidally-synchronized binaries. We see clear evidence for a transition between rapid and slow rotation on the subgiant branch in the domain predicted by modern angular momentum evolution models. We find substantial agreement between the spectroscopic and photometric properties of KIC targets added by Huber et al (2014) based on 2MASS photometry. For the unevolved lower main sequence, we see the same concentration toward rapid rotation in photometric binaries as that observed in rotation period data, but at an enhanced rate. We attribute this difference to unresolved near-equal luminosity spectroscopic binaries with velocity displacements on the order of the APOGEE resolution. Among cool unevolved stars we find an excess rapid rotator fraction of 4% caused by pipeline issues with photometric binaries.