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

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

An X-ray emitting black hole in a globular cluster

We present optical and X-ray data for the first object showing strong evidence for being a black hole in a globular cluster. We show the initial X-ray light curve and X-ray spectrum which led to the discovery that this is an extremely bright, highly variable source, and thus must be a black hole. We present the optical spectrum which unambiguously identifies the optical counterpart as a globular cluster, and which shows a strong, broad [O III] emission line, most likely coming from an outflow driven by the accreting source.Comment: 5 pages, 2 figures, to appear in the proceedings of IAUS 246, "Dynamical Evolution of Dense Stellar Systems", ed. Vesperini, Giersz and Sill

A far-ultraviolet variable with an 18-minute period in the globular cluster NGC 1851

We present the detection of a variable star with an 18.05 minute period in far-ultraviolet (FUV) images of the globular cluster NGC 1851 taken with the Hubble Space Telescope (HST). A candidate optical counterpart lies on the red horizontal branch or the asymptotic giant branch star of the cluster, but it is statistically possible that this is a chance superposition. This interpretation is supported by optical spectroscopt obtained with HST/STIS: the spectrum contains none of the strong emission lines that would be expected if the object was a symbiotic star (i.e. a compact accretor fed by a giant donor). We therefore consider two other possibilities for the nature of FUV variable: (i) an intermediate polar (i.e. a compact binary containing an accreting magnetic white dwarf), or (ii) an AM CVn star (i.e. an interacting double-degenerate system). In the intermediate polar scenario, the object is expected to be an X-ray source. However, no X-rays are detected at its location in ? 65 ksec of Chandra imaging, which limits the X-ray luminosity to LX ? 1032 erg s?1. We therefore favour the AM CVn interpretation, but a FUV spectrum is needed to distinguish conclusively between the two possibilities. If the object is an AM CVn binary, it would be the first such system known in any globular cluster