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; Hamburger
Sternwarte, University of Hamburg, Germany

Since autumn 2013, the robotic 1.2m telescope TIGRE (formerly the Hamburg Robotic
Telescope) is operating in Guanajuato (Mexico, and see poster by Gonzalez et al.). Regularly
using the same calibration stars for S-indices as did the Mount Wilson project (see Baliunas et
al. 1995), our R=20,000 HEROS echelle spectra have provided a decade of chromospheric
activity monitoring of the same ca. 100 solar-type stars, including a smaller number with
pronounced cycles (see Schröder et al. 2013) of periods of o(10yrs), which were already
observed at Mount Wilson since the 1970ies. Hence, on a timescale of half a century, we can
now use those same stars, first shown by Olin C. Wilson and his group to perform solar-type
activity cycles, to probe the stability of these cycles. This is motivated by the solar cycle
variability, which is evident, when comparing the last two, relatively weak maxima monitored by
TIGRE, with the Mount Wilson data from the 1970ies and 1980ies. Hence, we here show that
this is not a singular behavior. Strong cycles seem to be less variable, while weaker cycles like
the solar are natural to change between stronger and weaker performance. In the case of the
Sun this behavior is consistent with an o(century)-period as of the semi-regular Gleissberg
cycle, 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