HV2112, a thorne- Z˙ytkow object or a super asymptotic giant branch star

Abstract

The very bright red star HV2112 in the Small Magellanic Cloud could be a massive Thorne- Z˙ytkow object (T Z˙O), a supergiant-like star with a degenerate neutron core.With a luminosity of over 105 L⊙, it could also be a super asymptotic giant branch (SAGB) star, a star with an oxygen/neon core supported by electron degeneracy and undergoing thermal pulses with third dredge up. Both TZ˙Os 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 TZ˙O 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. Neither SAGB stars nor TZ˙Os are known to be able to synthesize their own calcium but it may be possible to produce it in the final stages of the process that forms a TZ˙O, when the degenerate electron core of a giant star is tidally disrupted by a neutron star. Hence, it is more likely, on a fine balance, that HV2112 is indeed a genuine TZ˙O.