Unravelling the Period Gap using LAMOST Chromospheric Activity Indices

TL;DR

This study investigates the origin of the period gap in BY Draconis stars by analyzing chromospheric activity indices derived from LAMOST spectra, revealing a link between magnetic activity, stellar age, and the period gap.

Contribution

It combines multi-survey data with spectroscopic analysis to connect chromospheric activity levels to the period gap in BY Dra stars, highlighting the role of core-envelope coupling.

Findings

Chromospheric activity decreases steeply in K-M stars up to 700-1000 Myr.

F-G stars show constant activity with age, indicating different evolution.

Low activity levels are associated with the period gap, suggesting a magnetic activity minimum.

Abstract

In our recent catalogue of BY Draconis (BY Dra) variables based on Zwicky Transient Facility data, we found traces of a period gap in the period-colour diagram. We combined our BY Dra database with catalogues from the {\sl Kepler} and K2 surveys, revealing a prominent period gap. Here, we use this combined ZTF-{\sl Kepler}-K2 data set to investigate the origin of the period gap observed for BY Dra stars using chromospheric activity indices. We use low- and medium-resolution spectra from the LAMOST Data Release 7 to derive magnetic activity indices for the Ca {\sc ii} H and K and H$\alpha$ emission lines. We find a strong dependence of chromospheric activity on both stellar mass and rotation period. For partially convective K-M-type stars, the activity decreases steeply up to an age of $\sim$700-1000 Myr, subsequently evolving to the type of low-level saturation associated with spin-down stallation. In contrast, F-G-type stars with thinner convective envelopes exhibit constant activity with increasing age. We suspect that the observed steep decrease for partially convective stars is driven by core-envelope coupling. This mechanism reduces differential rotation at the core-envelope transition, hence leading to decreased magnetic activity. Moreover, we derive activity indices for previously known star clusters and find similar trends as regards their activity levels as a function of age. In particular, very low-level activity is observed around the location of the period gap. Therefore, we conclude that the period gap, defined by the non-detection of variable sources, is driven by a minimum in chromospheric activity.