An ancient F-type subdwarf from the halo crossing the Galactic plane^⋆

¹ Leibniz Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: rdscholz@aip.de
² Dr. Remeis Observatory & ECAP, Astronomical Institute, Friedrich-Alexander University Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany

Received: 5 December 2014
Accepted: 19 December 2014

Abstract

Aims. We selected the bluest object, WISE J0725−2351, from Luhman’s new high proper motion (HPM) survey based on observations with the Wide-field Infrared Survey Explorer (WISE) for spectroscopic follow-up observations. Our aim was to unravel the nature of this relatively bright (V ~ 12, J ~ 11) HPM star (μ = 267 mas/yr).

Methods. We obtained low- and medium-resolution spectra with the European Southern Observatory (ESO) New Technology Telescope (NTT)/EFOSC2 and Very Large Telescope (VLT)/X-Shooter instruments, investigated the radial velocity and performed a quantitative spectral analysis that allowed us to determine physical parameters. The fit of the spectral energy distribution based on the available photometry to low-metallicity model spectra and the similarity of our target to a metal-poor benchmark star (HD 84937) allowed us to estimate the distance and space velocity.

Results. As in the case of HD 84937, we classified WISE J0725−2351 as sdF5: or a metal-poor turnoff star with [ Fe/H ] = −2.0 ± 0.2, T_eff = 6250 ± 100 K, log  g = 4.0 ± 0.2, and a possible age of about 12 Gyr. At an estimated distance of more than 400 pc, its proper motion translates to a tangential velocity of more than 500 km s^-1. Together with its constant (on timescales of hours, days, and months) and large radial velocity (about +240 km s^-1), the resulting Galactic restframe velocity is about 460 km s^-1, implying a bound retrograde orbit for this extreme halo object that currently crosses the Galactic plane at high speed.