Photometric Variability in Kepler Target Stars. III. Comparison with the Sun on Different Timescales

TL;DR

This study compares the photometric variability of the Sun and solar-type stars using Kepler data across various timescales, revealing the Sun's typical variability and the increasing activity in cooler stars, especially M dwarfs.

Contribution

It introduces a diagnostic for variability analysis, compares solar and stellar variability across timescales, and quantifies the active fraction of stars relative to the Sun, especially among cooler stars.

Findings

The Sun's variability is typical among solar-type stars.

A quarter to a third of stars are more active than the Sun depending on timescale.

Over 90% of early M dwarfs are more active than the Sun.

Abstract

We utilize Kepler data to study the precision differential photometric variability of solar-type and cooler stars at different timescales, ranging from half an hour to 3 months. We define a diagnostic that characterizes the median differential intensity change between data bins of a given timescale. We apply the same diagnostics to SOHO data that has been rendered comparable to Kepler. The Sun exhibits similar photometric variability on all timescales as comparable solar-type stars in the Kepler field (it is not unusually quiet). The previously-defined photometric "range" serves as our activity proxy (driven by starspot coverage). We revisit the fraction of comparable stars in the Kepler field that are more active than the Sun. The exact active fraction depends on what is meant by "more active than the Sun", and on the magnitude limit of the sample of stars considered. This active fraction is between a quarter and a third (depending on the timescale). We argue that a reliable result requires timescales of half a day or longer and stars brighter than Kepler magnitude of 14, otherwise non-stellar noise distorts it. We also analyze main sequence stars grouped by temperature from 6500-3500K. As one moves to cooler stars, the active fraction of stars becomes steadily larger (greater than 90% for early M dwarfs). The Sun is a good photometric model at all timescales for those cooler stars that have long-term variability within the span of solar variability.