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

Abstract

Stars with unusual properties can provide a wealth of information about rare stages of stellar evolution and exotic physics. However, determining the true nature of peculiar stars is often difficult. In this work, we conduct 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 HV 2112. Using light curves from ASAS-SN, we identify 38 stars with surface temperatures T<4800 K, luminosities log(L/L e)>4.3, variability periods >400 days, and variability amplitudes ΔV>2.5 mag. Eleven of these stars possess the distinctive double-peaked light-curve morphology of HV 2112. We use the pulsation properties and derived occurrence rates for these 12 objects to constrain their nature. From comparisons to stellar populations and models, we find that one star may be a red supergiant with large-amplitude pulsations. For the other 11 stars, we derive current masses of ∼5-10 M e , below the theoretical minimum mass of ∼15 M e for TŻOs to be stable, casting doubt on this interpretation. Instead, we find that the temperatures, luminosities, mass-loss rates (MLRs), and periods of these stars are consistent with predictions for super-asymptotic giant branch (s-AGB) stars that have begun carbon burning but have not reached the superwind phase. We infer lifetimes in this phase of ∼(1-7)×10 4 yr, also consistent with an s-AGB interpretation. If confirmed, these objects would represent the first identified population of s-AGB stars, illuminating the transition between low-and high-mass stellar evolution. Unified Astronomy Thesaurus concepts: Massive stars (732); Asymptotic giant branch stars (2100); Variable stars (1761); Chemically peculiar stars (226); Light curves (918); Photometry (1234)

Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds. II. Spectroscopic and Environmental Analysis of Thorne–Żytkow Object and Super-AGB Star Candidates

“…Motivation: Our goal in this paper was to clarify the nature of a population of sAGB star candidates identified by O'Grady et al (2020), and to see if any evidence supported a TŻO identity for any of the stars. We investigated their kinematics, local SFH, and radio properties.…”

Constraints on Weak Supernova Kicks from Observed Pulsar Velocities

“…Although no ECSN progenitors have been observationally confirmed, some candidate populations have been proposed which, if validated, would help to constrain the nature of ECSNe in single stars (O'Grady et al 2020). Hiramatsu et al (2021) propose that SN 2018zd was an ECSN from a single sAGB star based on the low energy and chemical profile of the light curve.…”

Properties of Thorne-Zytkow object explosions

“…Levesque et al (2014) identified a chemical anomaly in a supergiant star HV2112 in Small Magellanic Cloud (Worley et al 2016;McMillan & Church 2018) which is consistent with a TŻO (Tout et al 2014). It is also suggested that HV2112 may be an asymptotic-giant-branch star rather than a TŻO (Tout et al 2014;Beasor et al 2018;O'Grady et al 2020). There are ongoing observational efforts to discover TŻOs (DeMarchi et al 2021).…”