DASCH Discovery of Large Amplitude ~10-100 Year Variability in K Giants

Here we present the discovery of three unusual long-term variables found in the Digital Access to a Sky Century at Harvard project, with ~1 mag variations in their light curves on ~10-100 year timescales. They are all spectroscopically identified as K2III giant stars, probably in the thick disk. Their light curves do not match those previously measured for known types of variable stars, or any theoretical model reported for red giants, and instead suggest a new dust formation mechanism or the first direct observation of "short" timescale evolution-driven variability. More theoretical work on the lithium flash near the red giant branch bump and the helium shell ignition in the lower asymptotic giant branch, as well as long-term monitoring of K2III thick-disk stars is needed.