Large proper motion of the Thorne-\.Zytkow object candidate HV 2112 reveals its likely nature as foreground Galactic S-star

TL;DR

The paper demonstrates that HV 2112 is likely a foreground Galactic star with high proper motion, challenging its previous classification as a Thorne-Żytkow object in the SMC, and proposes a new binary scenario involving an extrinsic S star.

Contribution

It provides new proper motion measurements that suggest HV 2112 is a Galactic foreground star and introduces a novel binary scenario to explain its properties.

Findings

HV 2112 has a proper motion indicating it is a foreground star.

The star's properties are inconsistent with being in the SMC.

A binary scenario explains its spectral features and variability.

Abstract

Using the Southern Proper Motion (SPM) catalog, we show that the candidate Thorne-\.Zytkow object HV~2112 has a proper motion implying a space velocity of about 3000\kms if the object is located at the distance of the Small Magellanic Cloud. The proper motion is statistically different from that of the SMC at approximately $4\sigma$ in SPM, although the result can drop to about $3\sigma$ significance by including the UCAC4 data and considering systematic uncertainties in addition to the statistical ones. Assuming the measurement is robust, this proper motion is sufficient to exclude its proposed membership of the Small Magellanic Cloud and to argue instead that it is likely to be a foreground star in the Milky Way halo. The smaller distance and therefore lower brightness argue against its proposed nature as a Thorne-\.Zytkow object (the hypothesized star-like object formed when a normal star and a neutron star merge) or a super-Asymptotic Giant Branch (AGB) star. Instead we propose a binary scenario where this star is the companion of a former massive AGB star, which polluted the object with via its stellar wind, i.e. a special case of an extrinsic S star. Our new scenario solves two additional problems with the two existing scenarios for its nature as Thorne-\.Zytkow object or present-day super-AGB star. The puzzling high ratio of the strength of calcium to iron absorption lines is unexpected for SMC supergiants, but is fully consistent with the expectations for halo abundances. Secondly, its strong variability can now be explained naturally as a manifestation of the Mira phenomenon.