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Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds from ASAS-SN: Implications for Thorne-Żytkow Objects and Super-AGB Stars

Presentation #121.03 in the session “An Exploration of Thorne-Zytkow Objects: TZOs, Observations and Progenitors (Meeting-in-a-Meeting)”.

Published onJun 18, 2021
Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds from ASAS-SN: Implications for Thorne-Żytkow Objects and Super-AGB Stars

In this talk, I present the results of a systematic search for cool and luminous stars in the Magellanic Clouds with extreme variability, motivated by the properties of the unusual Small Magellanic Cloud star and Thorne-Żytkow Object (TŻO) candidate HV2112. Using light curves from the All-Sky Automated Survey for Supernovae, we identified 10 stars with similar photometric properties and light-curve morphology to that of HV2112. We used the pulsation properties, derived occurrence rates, and comparisons to stellar populations and models to constrain the nature of these 11 objects, including HV2112. Most importantly, we estimated current masses of ∼5-10 M☉, below the theoretical minimum mass of ∼15 M☉ required for TŻOs to be stable. We instead found that the photometric, variability, and derived physical properties of these stars are consistent with predictions for super-asymptotic giant branch (s-AGB) stars. If confirmed, these objects would represent the first identified population of s-AGB stars, illuminating the transition between low- and high-mass stellar evolution.


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