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

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.

6 pages, 3 figures. Accepted for publication in ApJ Letters. Typo corrected

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• A statistical analysis of the accuracy of the digitized magnitudes of photometric plates on the time scale of decades with an application to the century-long light curve of KIC 8462852
• Dissecting magnetar variability with Bayesian hierarchical models
• Magnitude-range brightness variations of overactive K giants
• DASCH on KU Cyg: a ~5 year dust accretion event in ~1900
• Long-term photometry of three active red giants in close binary systems: V2253 Oph, IT Com and IS Vir
• Discovering vanishing objects in POSS I red images using the Virtual Observatory
• The Stability of F-star Brightness on Century Timescales
• DASCH 100-yr light curves of high-mass X-ray binaries