Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds: Implications for Thorne-Zytkow Objects and Super-AGB Stars

The true identity of the peculiar star HV2112, located in the SMC, has been debated. Previous studies have identified it as the first strong Thorne-Zytkow Object (TZO) candidate — a class of cool, luminous stars with neutron star cores — while others have argued its properties are more consistent with massive or super-AGB (m/sAGB) stars, the latter of which are the most massive stars that do not undergo core collapse supernova. While these studies identified enhanced heavy element abundances in HV2112’s atmosphere, it also shows extreme variability (>4 mag), which is abnormal for stars with RSG-like luminosities. We characterized the variability of HV2112 and conducted a systematic search for cool and luminous stars in the Magellanic Clouds with similar variability properties. Using light-curves from ASAS-SN we identified 29 new candidates, nine of which also possess the distinctive light-curve morphology exhibited by HV2112. In this talk we present the results of our analysis of these stars. We find that the temperatures, luminosities, periods, and masses derived from pulsation properties of these sources are consistent with predictions for m/sAGBs, with HV2112 in particular falling into the mass range of the largest sAGBs, and below the theoretical minimum mass for TZOs. Subsequently, we have assessed the presence of heavy elements and lithium in their spectra, and used their local SFH, kinematic, and radio environments to place constraints on the age of progenitor systems and evidence for past supernovae. In all cases results are consistent with an mAGB or sAGB star identity, providing important information on the transition between low and high mass stellar evolution.