HD 166620: Portrait of a Star Entering a Grand Magnetic Minimum

TL;DR

This study provides conclusive evidence that HD 166620 is entering a grand magnetic minimum, similar to the Maunder Minimum, based on extended activity data, and discusses implications for identifying such states.

Contribution

The paper presents new long-term activity and photometry data confirming HD 166620's transition into a Maunder Minimum-like state, the first definitive case of such a star.

Findings

HD 166620 is entering a grand magnetic minimum.

Brightness and activity levels are not significantly lower during the minimum.

Reduced intraseasonal variability in activity may indicate a grand minimum.

Abstract

HD 166620 was recently identified as a Maunder Minimum candidate based on nearly 50 years of Ca II H & K activity data from Mount Wilson and Keck-HIRES (Baum et al. 2022). These data showed clear cyclic behavior on a 17-year timescale during the Mount Wilson survey that became flat when picked up later with Keck-HIRES planet-search observations. Unfortunately, the transition between these two data sets -- and therefore the transition into the candidate Maunder Minimum phase -- contained little to no data. Here we present additional Mount Wilson data not present in Baum et al. (2022) along with photometry over a nearly 30-year baseline that definitively trace the transition from cyclic activity to a prolonged phase of flat activity. We present this as conclusive evidence of the star entering a grand magnetic minimum and therefore the first true Maunder Minimum analog. We further show that neither the overall brightness nor the chromospheric activity level (as measured by S$_{\mathrm{HK}}$) is significantly lower during the grand magnetic minimum than its activity cycle minimum, implying that anomalously low mean or instantaneous activity levels are not a good diagnostic or criterion for identifying additional Maunder Minimum candidates. Intraseasonal variability in S$_{\mathrm{HK}}$, however, is lower in the star's grand minimum; this may prove a useful symptom of the phenomenon.