A Kink in the Spindown Road: Some Strange Dynamo Happenings Midway Between Saturation and the Sun

by Steven Saar (Harvard-Smithsonian Center for Astrophysics)

Tuesday 27 Apr 2021, 15:00 → 16:00 Europe/Stockholm

https://stockholmuniversity.zoom.us/j/940229961

For decades, rotational spindown and the associated decrease in magnetic activity have been thought to follow fairly straightforward, monotonic relationships: with the rotation period P ~ (age)^0.5, and activity fluxes F ~ (tau/P)^beta where tau is the convective turnover time and beta is a value increasing with the "formation temperature" of the emission under study.  Recently, though, large photometric surveys (yielding more periods) and deeper looks at old Ca HK and X-ray surveys have begun to uncover deviations from this pleasingly simple scenario.  I first review previous hints of this strange behavior, leading up to our discovery of a notable reversal - a kink - in the decrease of photometric amplitudes with increasing P, wherein amplitudes instead increase with increasing P for a time.  I compare this "kink" with other known happenings (some quite new!) in the evolution of cool dwarfs, and speculate on its origins. It would appear that something is changing in dynamos at this point, either making them more efficient at flux production, changing how the flux is distributed, or a bit of both.

 

Saar_21-04-27.mp4