HV2112, a Thorne-Zytkow Object or a Super Asymptotic Giant Branch Star

TL;DR

HV2112's unique calcium abundance suggests it is most likely a Thorne-Zytkow Object, a rare star with a neutron core, rather than a super asymptotic giant branch star, based on recent models and stellar processes.

Contribution

The paper proposes that HV2112 is most likely a Thorne-Zytkow Object, based on calcium abundance analysis and stellar formation models, distinguishing it from similar stellar types.

Findings

HV2112's calcium abundance supports TZO classification.

Heavy element abundances do not conclusively distinguish TZO from SAGB.

Final stages of TZO formation may produce observable calcium enhancements.

Abstract

The very bright red star HV2112 in the Small Magellanic Cloud could be a massive Thorne-Zytkow Object, a supergiant-like star with a degenerate neutron core. With its luminosity of over $10^5\,\rm L_\odot$, it could also be a super asymptotic giant branch star, a star with an oxygen/neon core supported by electron degeneracy and undergoing thermal pulses with third dredge up. Both TZOs and SAGB stars are expected to be rare. Abundances of heavy elements in HV2112's atmosphere, as observed to date, do not allow us to distinguish between the two possibilities based on the latest models. Molybdenum and rubidium can be enhanced by both the irp-process in a TZO or by the s-process in SAGB stars. Lithium can be generated by hot bottom burning at the base of the convective envelope in either. HV2112's enhanced calcium could thus be the key determinant. A SAGB star is not able to synthesise its own calcium but it may be possible to produce this in the final stages of the process that forms a TZO, when the degenerate electron core of a giant star is tidally disrupted by a neutron star. Hence our calculations indicate that HV2112 is most likely a genuine TZO.