Stellar Rotation in the K2 Sample: Evidence for Modified Spindown

TL;DR

This study analyzes K2 light curves to detect stellar rotation periods, confirming a rotation gap and suggesting a modified spin-down law, providing detailed rotation parameters for nearly 9,000 main-sequence stars.

Contribution

First robust detection of the rotation period gap in K2 data for field stars, indicating a departure from the classical spin-down law.

Findings

Confirmed the rotation period-color gap in K2 data.

Detected departure from the Skumanich spin-down law.

Provided detailed rotation parameters for thousands of stars.

Abstract

We analyze light curves of 284,834 unique K2 targets using a Gaussian process model with a quasi-periodic kernel function. By crossmatching K2 stars to observations from Gaia Data Release 2, we have identified 69,627 likely main-sequence stars. From these we select a subsample of 8,977 stars on the main-sequence with highly precise rotation period measurements. With this sample we recover the gap in the rotation period-color diagram first reported by \cite{McQuillan2013}. While the gap was tentatively detected in \cite{Reinhold2020}, this work represents the first robust detection of the gap in K2 data for field stars. This is significant because K2 observed along many lines of sight at wide angular separation, in contrast to Kepler's single line of sight. Together with recent results for rotation in open clusters, we interpret this gap as evidence for a departure from the $t^{-1/2}$ Skumanich spin down law, rather than an indication of a bimodal star formation history. We provide maximum likelihood estimates and uncertainties for all parameters of the quasi-periodic light curve model for each of the 284,834 stars in our sample.