Long-term Photometric Variability in Kepler Full-frame Images: Magnetic Cycles of Sun–like Stars

Abstract

Photometry from the Kepler mission is optimized to detect small, short duration signals like planet transits at the expense of long-term trends. This long-term variability can be recovered in photometry from the Full Frame Images (FFIs), a set of calibration data collected approximately monthly during the Kepler mission. Here, we present f3, an open-source package to perform photometry on the Kepler FFIs in order to detect changes in the brightness of stars in the Kepler field of view over long time baselines. We apply this package to a sample of 4,000 Sun-like stars with measured rotation periods. We find ≈ 10% of these targets have long-term variability in their observed flux. For the majority of targets we identify the luminosity variations are either correlated or anticorrelated with the short-term variability due to starspots on the stellar surface. We find a transition between anticorrelated (starspot-dominated) variability and correlated (facula-dominated) variability between rotation periods of 15 and 25 days, suggesting the transition between the two modes is complete for stars at the age of the Sun. We also identify a sample of stars with apparently complete cycles, as well as a collection of short-period binaries with extreme photometric variation over the Kepler mission.

Where Is the Flux Going? The Long-term Photometric Variability of Boyajian’s Star

“…3 of Boyajian et al 2016). Moreover, the increase in short-term photometric variability occurs simultaneously with the observed decrease in the star's brightness, similar to the correlation observed in Kepler observations of rapidly rotating sun-like stars (Montet et al 2017).…”

“…Just five of the 21 candidate lowamplitude variables from ASAS-SN (stars 2, 3, 4, 7, and 10) have Kepler long cadence light curves, and only star 7 shows significant short-term variability. We use the methodology described by Montet et al (2017) to search for longer-period variability of these stars in the Kepler full frame images (FFIs). All five stars display clear longterm trends (of smaller amplitude than seen in the ASAS-SN data), which we regard as possible stellar activity signals.…”

The Influence of Metallicity on Stellar Differential Rotation and Magnetic Activity

“…The uncertainties on the relative photometry on the FFIs are due to a number of noise sources, where the most important are: photon noise, variability of comparison stars and inter-and intra-pixel sensitivity changes (Montet et al 2017). For HD 173701 the largest contribution is likely to come from percent-level inter-and intra-pixel sensitivity changes across the detector that can induce artificial offsets when the spacecraft changes orientation every quarter.…”

“…The photometric uncertainties are calculated from the quadratic fit as described in Montet et al (2017). This approach assumes that HD 173701, which is heavily saturated in the FFIs, behaves similar to the non-saturated reference stars.…”

“…We use the f3 software package described in Montet et al (2017) to infer the brightness of HD 173701 and 15 bright, nearby comparison stars in 52 FFIs spanning the Kepler mission. All target stars fall within 15 of HD 173701.…”