First direct detection of an RR Lyrae star conclusively associated with an intermediate-age cluster

Abstract

Context. RR Lyrae stars have long been considered unequivocal tracers of old (>10 Gyr) and metal-poor ([Fe/H] < −0.5) stellar populations. First, because these populations is where they are readily found and because, according to canonical stellar evolution models for isolated stars, these are the only populations where RR Lyrae are expected to exist. However, recent independent results are challenging this view and pointing to the existence of intermediate-age RR Lyrae (i.e. only about 2-5 Gyr old). Aims. Our goal in this work is to provide direct evidence of the existence of intermediate-age RR Lyrae by searching for these stars in Milky Way open clusters, where the age association would be direct and robust. Methods. We searched a catalogue of over 3000 open clusters with published kinematically associated member stars by cross-matching it against a compilation of the largest publicly available RR Lyrae surveys (Gaia, ASAS-SN, Pan-STARRS1, Zwicky Transient Facility, and OGLE-IV). Results. We identified a star as a bona fide RR Lyrae variable and robust member of the 2-4 Gyr old Trumpler 5 cluster, based on its parallax and proper motions and their agreement with confirmed cluster members. We derived an extremely low probability (0.049 ± 0.013%) of the star being a background field RR Lyrae and we provide initial constraints on a possible binary companion based on its position in the colour-absolute-magnitude diagram. Conclusions. As a current source of debate, the Trumpler 5 RR Lyrae star provides the most direct evidence to date of the existence of RR Lyrae stars at much younger ages than traditionally expected, adding to the mounting evidence supporting their existence.