Long-Term Changes in the Variability of Pulsating Red Giants (and One R CrB Star)

Abstract

We have used many decades of visual observations from the AAVSO International Database, and the AAVSO time-series analysis package VSTAR, to study the long-term changes in period, amplitude, and mean magnitude in about 30 normal pulsating red giants (PRGs) i.e. those without large secular changes in period, as well as a few of the rare PRGs which do have such secular period changes. The periods of the typical PRGs "wander" on time scales of about 40 pulsation periods -- significantly longer than the time scales of amplitude variation which are 20-35 with a mean of 27 pulsation periods. We have also studied the range and time scale of the long-term changes in pulsation amplitude and mean magnitude, as well as period, and looked for correlations between these. Changes in mean magnitude are larger in stars in stars with larger mean amplitude, but correlate negatively with changes in amplitude. There is a weak positive correlation between the long-term period changes and amplitude changes. The causes of these three kinds of long-term changes are still not clear. We note, from the presence of harmonics in the Fourier spectra, that the longest-period PRGs have distinctly non-sinusoidal phase curves. For studying PRGs, we demonstrate the advantage of studying stars with minimal seasonal gaps in the observations, such as those near the celestial poles. We studied Z UMi, misclassified as a possible Mira star, but actually an R CrB star. We determined times of onset of 16 fadings. We were not able to determine a coherent pulsation period for this star at maximum, with an amplitude greater than 0.05. We did, however, find that the times of onset of fadings were "locked" to a 41.98-day period -- a typical pulsation period for an R CrB star.