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

<p>&nbsp;<br>by Klaus-Peter Schr&ouml;der (1); J&uuml;rgen Schmitt (2); Marco Mittag (2)<br>Departamento de Astronomia, Universidad de Guanajuato, Mexico; Hamburger<br>Sternwarte, University of Hamburg, Germany</p> <p>Since autumn 2013, the robotic 1.2m telescope TIGRE (formerly the Hamburg Robotic<br>Telescope) is operating in Guanajuato (Mexico, and see poster by Gonzalez et al.). Regularly<br>using the same calibration stars for S-indices as did the Mount Wilson project (see Baliunas et<br>al. 1995), our R=20,000 HEROS echelle spectra have provided a decade of chromospheric<br>activity monitoring of the same ca. 100 solar-type stars, including a smaller number with<br>pronounced cycles (see Schr&ouml;der et al. 2013) of periods of o(10yrs), which were already<br>observed at Mount Wilson since the 1970ies. Hence, on a timescale of half a century, we can<br>now use those same stars, first shown by Olin C. Wilson and his group to perform solar-type<br>activity cycles, to probe the stability of these cycles. This is motivated by the solar cycle<br>variability, which is evident, when comparing the last two, relatively weak maxima monitored by<br>TIGRE, with the Mount Wilson data from the 1970ies and 1980ies. Hence, we here show that<br>this is not a singular behavior. Strong cycles seem to be less variable, while weaker cycles like<br>the solar are natural to change between stronger and weaker performance. In the case of the<br>Sun this behavior is consistent with an o(century)-period as of the semi-regular Gleissberg<br>cycle, as well as with the occurrence of grand-minima.</p> <p><br>Theme(s): The Sun and Cool Stars in the Time Domain, Cool Stars as Stellar Systems</p>