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

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.

Lithium abundance in a turnoff halo star on an extreme orbit

“…The difference between the value of [Fe/H] in Table 1 and the value obtained by Scholz et al (2015) is mainly due to the difference in the adopted temperatures. WISE J0725-2351 is located on the metal-rich edge of the interval of metallicity studied during the ESO Large Programme "First Stars" (Cayrel et al 2004;Bonifacio et al 2009).…”

“…The theory of Barklem et al (2000) A&A 582, A74 (2015) is used here, via turbospectrum, to treat the hydrogen self-broadening, whereas (Scholz et al 2015) use the grid of synthetic spectra of Munari et al (2005) based on the theory of Ali & Griem (1966) : this theory is known to lead to higher temperatures. We computed theoretical profiles of the H α line with three values of T eff : 5750 K, 6000 K, and 6250 K. The best fit is obtained for a temperature of 6050 K; see Fig.…”

“…Recently, Scholz et al (2015) have studied a new high proper motion metal-poor turnoff star, with μ = 267 mas/yr selected from a high proper motion survey (Luhman 2014), based on observations with the Wide-field Infrared Survey Explorer (WISE; Wright et al 2010).…”

“…Their findings indicate that the star has a high radial velocity, and thus a resulting Galactic rest frame velocity of about 460 km s −1 , showing that WISE J0725-2351 belongs to the extreme halo and crosses the Galactic disk on a retrograde bound orbit approaching the Sun at a distance of 400 pc. Scholz et al (2015) obtained spectra with the mediumresolution spectrograph X-Shooter mounted on the ESO Very Large Telescope and deduced a first estimation of the atmospheric parameters of the star, T eff = 6250 K, log g = 4.0, [Fe/H] = -2.0, via a comparison with the observed spectrum to a grid of theoretical spectra based on Kurucz models (Castelli & Kurucz 2004;Munari et al 2005) temperatures, gravities, and metallicities. The low metallicity of the star shows that it pertains to the old population of the Galaxy.…”

“…The low metallicity of the star shows that it pertains to the old population of the Galaxy. Scholz et al (2015) also compared the spectrum of WISE J0725-2351 with those of two classical Population II stars, HD 84937 and HD 140283, and found that the spectra are similar and that these stars should be also comparable in age. The age of HD 140283 is estimated to be close to the age of the universe following VandenBerg et al (2014).…”