T4 APT photometry of HD 166620

These data showed clear cyclic behavior on a 17yr 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., along with photometry over a nearly 30yr baseline that definitively traces 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 SHK) is significantly lower during the grand magnetic minimum than its activity cycle minimum, implying that an anomalously low mean or instantaneous activity levels are not a good diagnostic or criterion for identifying additional Maunder minimum candidates.