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

Abstract

Using the Southern Proper Motion (SPM) catalog, we show that the candidate Thorne-Żytkow 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-Żytkow 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-Żytkow 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.

5 pages, 2 figures, accepted to MNRAS Letters, replaced tex file to make figure display properly, no changes to text

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• A dearth of OH/IR stars in the Small Magellanic Cloud
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• The proper motion of HV2112: A TŻO candidate in the SMC
• Rethinking Thorne-Żytkow Object Formation: The Fate of X-ray Binary LMC X-4 and Implications for Ultra-long Gamma-ray Bursts
• Blue Supergiant X-ray Binaries in the Nearby Dwarf Galaxy IC 10
• Population synthesis of Thorne-Żytkow objects: Rejuvenated donors and unexplored progenitors in the common envelope formation channel
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