A Very Metal-poor RR Lyrae Star with a Disk Orbit Found in the Solar Neighborhood

TL;DR

This paper reports the discovery of a very metal-poor RR Lyrae star in the solar neighborhood with disk-like kinematics, providing insights into early star formation in the Galaxy.

Contribution

It presents the identification of the most metal-deficient RR Lyrae star with disk kinematics, expanding understanding of metal-poor stars in the Galactic disk.

Findings

Star has [Fe/H] < -2.5, extremely metal-poor.

Located about 1 kpc from the Sun in the thick disk.

Potential to inform early Galactic star formation studies.

Abstract

Metal-deficient stars are important tracers for understanding the early formation of the Galaxy. Recent large-scale surveys with both photometric and spectroscopic data have reported an increasing number of metal-deficient stars whose kinematic features are consistent with those of the disk stellar populations. We report the discovery of an RR~Lyrae variable (hereafter RRL) that is located within the thick disk and has an orbit consistent with the thick-disk kinematics. Our target RRL (HD 331986) is located at around 1 kpc from the Sun and, with V=11.3, is among the 130 brightest RRLs known so far. However, this object was scarcely studied because it is in the midplane of the Galaxy, the Galactic latitude around -1 deg. Its near-infrared spectrum (0.91-1.32 micron) shows no absorption line except hydrogen lines of the Paschen series, suggesting [Fe/H] less than -2.5. It is the most metal-deficient RRL, at least, among the RRLs whose orbits are consistent with the disk kinematics, although we cannot determine to which of the disk and the halo it belongs. This unique RRL would provide us with essential clues for studying the early formation of stars in the inner Galaxy with further investigations, including high-resolution optical spectroscopy.