Presentation #1043 in the session “Open Engagement Session A”.
It has been recently claimed that the Sun’s activity is abnormally low relative to analogous stars observed by NASA’s Kepler space telescope (Reinhold et al. 2020). Stars were identified that have temperatures, luminosities, and rotation periods that are similar to the modern Sun. Using the range of photometric variability as a proxy for magnetic activity (Rvar), the activity levels of stars in this sample reach values much higher than that of the contemporary Sun. The implication is that the Sun’s magnetic dynamo (and associated phenomena like variations in total solar irradiance, high-energy emission, and flaring events) could also exhibit much higher excursions in the future or the recent past.
However, the primary conclusion of the Reinhold et al. study is due to a biased sample of solar twins stemming from an erroneous temperature scale, rather than an abnormally inactive Sun. The temperatures used to assemble the solar analog sample are biased toward warmer values by ~200 K, resulting in the sample actually being composed of early-K and late-G dwarfs instead of true solar analogs. Since magnetic activity tightly correlates with Rossby number, it makes sense that such stars are naturally more active while spinning at the solar rate due to their larger convective zones, longer convective turnover times, and therefore smaller Rossby numbers. Compiling a list of true solar analogs observed by Kepler is challenging due to the fact that their rotation periods are difficult to measure in broadband optical light. However, the few stars that can be curated have activity levels on par with the Sun’s present day range. This is consistent with other results from stellar astrophysics; for example, the chromospheric activity survey of solar twin stars in the 4-Gyr-old M67 cluster found activity levels consistent with the Sun’s contemporary range.