Letter to the Editor

No country for old stars

Spectroscopic confirmation of the first intermediate-age RR Lyrae in the open cluster Trumpler 5

¹ Department of Physics, University of Rome Tor Vergata, via della Ricerca Scientifica 1, I-00133 Rome, Italy
² INAF – Osservatorio Astronomico di Roma, Via Frascati 33, Monte Porzio Catone I-00078, Italy
³ Departamento de Astronomía, Instituto de Física, Universidad de la República, Iguá 4225, CP 11400 Montevideo, Uruguay
⁴ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona, c. Martí i Franquès 1, 08028 Barcelona, Spain
⁵ School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
⁶ Australian Research Council Centre of Excellence for Gravitational Wave Discovery, Clayton, VIC 3800, Australia
⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany
⁸ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland
⁹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via P. Gobetti 93/3, I-40129 Bologna, Italy
¹⁰ Institute of Astronomy, Russian Academy of Sciences, 119017 Moscow, Russia
¹¹ INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, I-35122 Padova, Italy
¹² Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany
¹³ Gemini Observatory/NSF’s NOIRLab, Casilla 603, La Serena, Chile

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 April 2026
Accepted: 10 April 2026

Abstract

Context. RR Lyrae (RRL) stars are widely considered tracers of ancient (> 10 Gyr) metal-poor stellar populations. However, recent kinematic and photometric studies suggest the existence of a metal-rich RRL subpopulation associated with the thin disk and intermediate ages (∼2–5 Gyr), therefore challenging canonical evolutionary models.

Aims. We aim to provide the first spectroscopic confirmation of a member of this elusive population. Specifically, we target a metal-rich RRL candidate recently identified photometrically as a member of the intermediate-age open cluster Trumpler 5 (∼2.5 Gyr).

Methods. We obtained high-resolution spectroscopy using PEPSI at the LBT and GHOST at Gemini South telescope. We measured radial velocities (RVs) from multiple epochs to constrain cluster membership and derived detailed chemical abundances (Mg, Ca, Sc, Ti, Mn, Fe, Cu, Zn, Y, and Ba) to compare the RRL’s composition with that of red clump stars in the cluster.

Results. The RRL’s systemic velocity (V_γ = 50.57^+0.78_−0.36 km s⁻¹) is in excellent agreement with the cluster mean (V = 50.76 ± 0.49 km s⁻¹). Combining RVs, proper motions, and parallax, the probability of the star being a background interloper is negligible (∼0.002%, better than 4σ). We derived a metallicity of [Fe/H] = −0.40 ± 0.05, which matches the cluster value. While most abundance ratios (Mg, Ti, Mn, Cu, and Zn) align with cluster members, the RRL exhibits significant depletion in Ca, Sc, Y, and Ba. Notably, [Sc/Fe] is underabundant by ∼0.6 dex relative to the cluster stars, following trends seen in field metal-rich RRLs.

Conclusions. We provide strong constraints on the membership status between an RRL variable and an intermediate-age open cluster. Cluster membership enables accurate measurement of RRL age and chemical anomalies relative to its host, particularly in Sc and neutron-capture elements. These anomalies further reinforce a nonstandard formation channel for this RRL, possibly indicating binary interactions and mass transfer.