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

6 pages, 1 figure, submitted to ApJL

References in corpus (4)

• Patterns of photometric and chromospheric variation among Sun-like stars: A 20-year perspective
• When Do Stalled Stars Resume Spinning Down? Advancing Gyrochronology with Ruprecht 147
• HARPS-N Solar Radial-Velocity Variations Are Dominated By Large, Bright Magnetic Regions
• Five Decades of Chromospheric Activity in 59 Sun-like Stars and New Maunder Minimum Candidate HD 166620