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

TL;DR

This paper reports the discovery of three long-term variable K giant stars with unusual brightness changes over 10-100 years, suggesting new variability mechanisms or dust formation processes in red giants.

Contribution

It presents the first identification of long-term variability in K giants that do not match existing models, indicating potential new physical processes.

Findings

Discovered three K2III giants with ~1 magnitude variability over decades.

Lightcurves do not match known variable star types or models.

Suggests possible new dust formation or evolutionary variability mechanisms.

Abstract

Here we present the discovery of three unusual long-term variables found in the Digital Access to a Sky Century at Harvard (DASCH) project, with ~1 magnitude variations in their lightcurves on ~10-100 yr timescales. They are all spectroscopically identified as K2III giant stars, probably in the thick disk. Their lightcurves do not match any 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 (RGB) bump and the helium shell ignition in the lower Asymptotic Giant Branch (AGB), as well as long term monitoring of K2III thick disk stars is needed.