Bridging the gap -- the disappearance of the intermediate period gap for fully convective stars, uncovered by new ZTF rotation periods

TL;DR

This study uses new ZTF rotation period data for a large sample of stars, revealing that the intermediate period gap observed in earlier surveys disappears at the fully convective boundary, supporting core-envelope interaction theories.

Contribution

The paper provides a new, extensive rotation period catalog for M dwarfs, developed with an improved pipeline, and demonstrates the disappearance of the period gap at the fully convective boundary.

Findings

The intermediate period gap closes at the fully convective boundary.

The new catalog contains over 40,000 rotation periods, including many longer than 10 days.

Evidence suggests rapid spin-down of stars across the period gap.

Abstract

The intermediate period gap, discovered by Kepler, is an observed dearth of stellar rotation periods in the temperature-period diagram at $\sim$ 20 days for G dwarfs and up to $\sim$ 30 days for early-M dwarfs. However, because Kepler mainly targeted solar-like stars, there is a lack of measured periods for M dwarfs, especially those at the fully convective limit. Therefore it is unclear if the intermediate period gap exists for mid- to late-M dwarfs. Here, we present a period catalog containing 40,553 rotation periods (9,535 periods $>$ 10 days), measured using the Zwicky Transient Facility (ZTF). To measure these periods, we developed a simple pipeline that improves directly on the ZTF archival light curves and reduces the photometric scatter by 26%, on average. This new catalog spans a range of stellar temperatures that connect samples from Kepler with MEarth, a ground-based time domain survey of bright M-dwarfs, and reveals that the intermediate period gap closes at the theoretically predicted location of the fully convective boundary ($G_{\rm BP} - G_{\rm RP} \sim 2.45$ mag). This result supports the hypothesis that the gap is caused by core-envelope interactions. Using gyro-kinematic ages, we also find a potential rapid spin-down of stars across this period gap.