The elderly among the oldest: new evidence for extremely metal-poor RR Lyrae stars^⋆

¹ Department of Physics, University of Rome Tor Vergata, via della ricerca scientifica 1, 00133 Rome, Italy
² INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ INAF – Osservatorio Astronomico di Roma, via Frascati 33, Monte Porzio Catone, Italy
⁴ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁵ INAF – Osservatorio Astronomico d’Abruzzo, via Maggini s/n, I-64100 Teramo, Italy
⁶ Department of Astronomy and McDonald Observatory, The University of Texas, Austin, TX 78712, USA
⁷ IAC – Instituto de Astrofísica de Canarias, calle Via Láctea s/n, E-38205 La Laguna, Tenerife, Spain
⁸ Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
⁹ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield AL10 9AB, UK
¹⁰ Materials Science and Applied Mathematics, Malmö University, SE-205 06 Malmö, Sweden
¹¹ Research School of Astronomy & Astrophysics, Australian National University, ACT 2611, Australia
¹² Center of Excellence for Astrophysics in Three Dimensions (ASTRO-3D), Australia
¹³ Physics Department, Technion – Israel Institute of Technology, Haifa 32000, Israel
¹⁴ Departament de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain
¹⁵ Department of Astronomy, School of Science,The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
¹⁶ Laboratory of Infrared High-resolution spectroscopy (LiH), Koyama Astronomical Observatory, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japan
¹⁷ INAF – Osservatorio Astronomico di Capodimonte, salita Moiariello 16, I-80131 Napoli, Italy
¹⁸ Department of Physics, Florida State University, 77 Chieftain Way, Tallahassee FL32306, USA
¹⁹ International Gemini Observatory/NSF NOIRLab, 670 N. A’ohoku Place, Hilo, Hawai’i 96720, USA
²⁰ Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37240, USA
²¹ Nishi-Harima Astronomical Observatory, Center for Astronomy, University of Hyogo, 407-2, Nishigaichi, Sayo-cho, Hyogo 679-5313, Japan
²² INAF – Osservatorio Astrofisica e Scienza dello Spazio, via Gobetti 93/3, Bologna, Italy

^⋆⋆ Corresponding author; vdorazi@roma2.infn.it

Received: 28 November 2024
Accepted: 6 January 2025

Abstract

We performed a detailed spectroscopic analysis of three extremely metal-poor RR Lyrae stars, exploring uncharted territories at these low metallicities for this class of stars. Using high-resolution spectra acquired with HARPS-N at TNG, UVES at VLT, and PEPSI at LBT, and employing Non-Local Thermodynamic Equilibrium (NLTE) spectral synthesis calculations, we provide abundance measurements for Fe, Al, Mg, Ca, Ti, Mn, and Sr. Our findings indicate that the stars have metallicities of [Fe/H] = −3.40 ± 0.05, −3.28 ± 0.02, and −2.77 ± 0.05 for HD 331986, DO Hya, and BPS CS 30317-056, respectively. Additionally, we derived their kinematic and dynamical properties to gain insights into their origins. Interestingly, the kinematics of one star (HD 331986) is consistent with the Galactic disc, while the others exhibit Galactic halo kinematics, albeit with distinct chemical signatures. We compared the [Al/Fe] and [Mg/Mn] ratios of the current targets with recent literature estimates to determine whether these stars were either accreted or formed in situ, finding that the adopted chemical diagnostics are ineffective at low metallicities ([Fe/H] ≲ −1.5). Finally, the established horizontal branch evolutionary models, indicating that these stars arrive at hotter temperatures on the Zero-Age Horizontal Branch (ZAHB) and then transition into RR Lyrae stars as they evolve, fully support the existence of such low-metallicity RR Lyrae stars. As a consequence, we can anticipate detecting more of them when larger samples of spectra become available from upcoming extensive observational campaigns.