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

In previous work, we identified a population of 38 cool and luminous variable stars in the Magellanic Clouds and examined 11 in detail in order to classify them as either Thorne-Zytkow objects (TZOs; red supergiants with a neutron star cores) or super-asymptotic giant branch (sAGB) stars (the most massive stars that will not undergo core collapse). This population includes HV 2112, a peculiar star previously considered in other works to be either a TZO or high-mass asymptotic giant branch (AGB) star. Here we continue this investigation, using the kinematic and radio environments and local star formation history of these stars to place constraints on the age of the progenitor systems and the presence of past supernovae. These stars are not associated with regions of recent star formation, and we find no evidence of past supernovae at their locations. Finally, we also assess the presence of heavy elements and lithium in their spectra compared to red supergiants. We find strong absorption in Li and s-process elements compared to RSGs in most of the sample, consistent with sAGB nucleosynthesis, while HV 2112 shows additional strong lines associated with TZO nucleosynthesis. Coupled with our previous mass estimates, the results are consistent with the stars being massive (similar to 4-6.5 M (circle dot)) or sAGB (similar to 6.5-12 M (circle dot)) stars in the thermally pulsing phase, providing crucial observations of the transition between low- and high-mass stellar populations. HV 2112 is more ambiguous; it could either be a maximally massive sAGB star, or a TZO if the minimum mass for stability extends down to less than or similar to 13 M (circle dot).

Automated summary

In this study, a population of 38 cool and luminous variable stars in the Magellanic Clouds was investigated, and 11 of them were examined in detail to classify them as either Thorne-Zytkow objects or super-asymptotic giant branch stars. These stars are not associated with recent star formation and there is no evidence of past supernovae at their locations.