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arXiv · 2507.17101

Ancient, eclipsing, tidally-locked: A blue lurker progenitor in the population of extreme-velocity star candidates

A. Bhat·M. Dorsch·S. Geier·U. Heber·H. Dawson·F. Mattig·D. Benitez-Palacios·Pablo Fernandez-Schlosser
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Abstract

Many extreme velocity candidate stars have been found based on \textit{Gaia} astrometry, but need spectroscopic confirmation. We select late-type hypervelocity star (HVS) candidates from the \textit{Gaia} DR3 catalog with a $1σ$ lower limit of the tangential velocity of 800 km\,s$^{-1}$. J1903-0023, one of the brightest targets, stands out as high priority candidate for follow-up spectroscopy using the X-shooter instrument at ESO-VLT. We determine its atmospheric parameters and abundances utilizing synthetic spectral grids and a global $χ^2-$minimization procedure, and its stellar parameters with the help of evolutionary tracks and the spectral energy distribution. The star shows variability in its light curve and follow-up spectroscopy confirms that the star is radial-velocity variable. The spectroscopic distance of J1903-0023 is lower than that based on the parallax, indicating that the star is not a hypervelocity binary star but bound to the Galaxy. The star turned out to be of spectral type F, very similar to the extreme-velocity star J0725-2351, which we analyse in the same way as the target. Apparently, both stars are very metal poor and old halo main-sequence (sdF) stars with masses slightly below the halo turn-off mass, and share the low metallicity ([Fe/H]=-2.3,-2.6) and strong alpha enhancement ([$α$/Fe]$\sim0.44$). While J0725-2351 is non-rotating ($v\sin\,i<3$\,km\,s$^{-1}$), J1903-0023 is a fast rotator ($v\sin i=42.3\pm2.0$ km\,s$^{-1}$). The Gaia, and ZTF light curves show an eclipse at a 1.179 day period, similar to the rotation period of J1903-0023. We therefore conclude that J1903-0023 is a high-velocity tidally-synchronised binary most likely with a metal-poor M dwarf companion.

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A. Bhat, M. Dorsch, S. Geier, U. Heber, H. Dawson, F. Mattig, D. Benitez-Palacios, Pablo Fernandez-Schlosser. 2025-07-24. Ancient, eclipsing, tidally-locked: A blue lurker progenitor in the population of extreme-velocity star candidates. https://doi.org/10.1051/0004-6361%2F202553995

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