How stable are solar-type cycles over half a century? Chromospheric S-indices Mt. Wilson vs. TIGRE

by Klaus-Peter Schröder (1); Jürgen Schmitt (2); Marco Mittag (2)Departamento de Astronomia, Universidad de Guanajuato, Mexico; HamburgerSternwarte, University of Hamburg, Germany

Since autumn 2013, the robotic 1.2m telescope TIGRE (formerly the Hamburg RoboticTelescope) is operating in Guanajuato (Mexico, and see poster by Gonzalez et al.). Regularlyusing the same calibration stars for S-indices as did the Mount Wilson project (see Baliunas etal. 1995), our R=20,000 HEROS echelle spectra have provided a decade of chromosphericactivity monitoring of the same ca. 100 solar-type stars, including a smaller number withpronounced cycles (see Schröder et al. 2013) of periods of o(10yrs), which were alreadyobserved at Mount Wilson since the 1970ies. Hence, on a timescale of half a century, we cannow use those same stars, first shown by Olin C. Wilson and his group to perform solar-typeactivity cycles, to probe the stability of these cycles. This is motivated by the solar cyclevariability, which is evident, when comparing the last two, relatively weak maxima monitored byTIGRE, with the Mount Wilson data from the 1970ies and 1980ies. Hence, we here show thatthis is not a singular behavior. Strong cycles seem to be less variable, while weaker cycles likethe solar are natural to change between stronger and weaker performance. In the case of theSun this behavior is consistent with an o(century)-period as of the semi-regular Gleissbergcycle, as well as with the occurrence of grand-minima.

Theme(s): The Sun and Cool Stars in the Time Domain, Cool Stars as Stellar Systems