No country for old stars -Spectroscopic confirmation of the first intermediate-age RR Lyrae in the open cluster Trumpler 5

Abstract

RR Lyrae stars are widely considered tracers of ancient (greater than 10 Gyr), metal-poor stellar populations. However, recent kinematic and photometric studies suggest the existence of a metal-rich RRL sub-population associated with the thin disc and intermediate ages (approximately 2-5 Gyr), challenging canonical evolutionary models. 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. We obtained high-resolution spectroscopy using PEPSI at the LBT and GHOST at Gemini-South Telescope. We measured radial velocities 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. The RRL's systemic velocity Vgamma = 50.57 +0.78/-0.36 km/s is in excellent agreement with the cluster mean V = 50.76 +/- 0.49 km/s. Combining RVs, proper motions, and parallax, the probability of the star being a background interloper is negligible (approximately 0.002%, better than 4 sigma). We derived a metallicity of [Fe/H] = -0.40 +/- 0.05, matching 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 under-abundant by approximately 0.6 dex relative to the cluster stars, following trends seen in field metal-rich RRLs. We provide strong constraints on the membership status between an RRL variable and an intermediate-age open cluster [...]