Solar Twins and Stellar Maunder Minima

In 1966, Olin C. Wilson undertook an answer to the question “Does the chromospheric activity of main-sequence stars vary with time, and if so, how?”, initiating the so-called HK Project at Mount Wilson Observatory, which resulted in a magnificent 43-year data set and which has spawned a number of complementary synoptic programs in both hemispheres. Subsequent developments, in particular the realization that activity controls angular momentum evolution in the stars and Sun, that solar activity modulates irradiance, and that there was a pronounced response of terrestrial climate to the Maunder Minimum, spurred efforts to identify solar twins, stars that Giusa Cayrel de Strobel required to possess “fundamental physical parameters very similar, if not identical to those of the Sun.” Non-cycling states appear to occur in the Mount Wilson stars and in other synoptic data with about the same frequency that the Sun’s grand minima occur in the long-term proxy record, suggesting that stellar analogs of the Maunder Minimum may be used to guide understanding of the Sun’s state in the late seventeenth century and, as appears possible given the extended Cycle 23/24 minimum, in the near future. However, the magnitude limits of the existing surveys have kept the sample of solar twins small and long-term monitoring programs have only recently begun to accumulate good time-domain data beyond the canonical HK-index. Addressing these and other issues toward understanding prolonged stellar minima is therefore a key area of inquiry in solar-stellar connection work for the next decade. I will summarize the state of the field and the most promising lines of work for the immediate future. I and my colleagues Wes Lockwood and Brian Skiff sincerely appreciate the National Science Foundation’s long-time support of stellar cycles work at Lowell Observatory.