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

Many extreme velocity candidate stars have been found based on Gaia astrometry, but need spectroscopic confirmation. We selected late-type hypervelocity star (HVS) candidates from the Gaia DR3 catalogue with a 1σ lower limit of the tangential velocity of 800 km s−1. J1903-0023, one of the brightest targets, stands out as a high-priority candidate for follow-up spectroscopy using the X-shooter instrument at ESO-VLT. We determined its atmospheric parameters and abundances utilising synthetic spectral grids and a global χ2-minimisation 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 is 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 analysed in the same way as the target. Apparently, both are very metal-poor and old halo main-sequence (sdF) stars with masses slightly below the halo turn-off mass, and they both show low metallicity ([Fe/H] = −2.3, −2.6) and strong alpha enhancement ([α/Fe]∼0.44). J0725-2351 is non-rotating (v sin i < 3 km s−1); instead, J1903-0023 is a fast rotator (v sin i = 42.3 ± 2.0 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.