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

Noriyuki Matsunaga, Akinori Itane, Kohei Hattori, Juliana Crestani, Vittorio Braga, Giuseppe Bono, Daisuke Taniguchi, Junichi Baba, Hiroyuki Maehara, Nobuharu Ukita, Tsuyoshi Sakamoto, Naoto Kobayashi, Tsutomu Aoki, Takao Soyano, Ken'Ichi Tarusawa, Yuki Sarugaku, Hiroyuki Mito, Shigeyuki Sako, Mamoru Doi, Yoshikazu NakadaNatsuko Izumi, Yoshifusa Ita, Hiroki Onozato, Mingjie Jian, Sohei Kondo, Satoshi Hamano, Chikako Yasui, Takuji Tsujimoto, Shogo Otsubo, Yuji Ikeda, Hideyo Kawakita

Research output: Contribution to journalArticlepeer-review


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 has scarcely been studied because it is in the midplane of the Galaxy, at a Galactic latitude around -1 . Its near-infrared spectrum (0.91-1.32 μm) shows no absorption line except hydrogen lines of the Paschen series, suggesting [Fe/H] ≲ -2.5. It is the most metal-deficient RRL, at least among 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.

Original languageEnglish
Article number10
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 20 Jan 2022
Externally publishedYes

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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