RR Lyrae (RRL) stars are commonly regarded as tracers ofancient (≳10 Gyr), metal-poor stellar populations, in line with predictions from canonical single-star evolutionary models. Nevertheless, recent studies have suggested the presence of RRL stars associated with younger populations, although direct age constraints based on secure associations with well-dated systems remain scarce.
In this talk, I will present new results centred on the Galactic open cluster Trumpler 5, an intermediate-age cluster with an estimated age of ∼2–4 Gyr. I will describe the identification of an RRL star whose Gaia astrometric parameters are consistent with those of confirmed cluster members and whose location in the colour–absolute magnitude diagram is compatible with cluster membership. A quantitative evaluation of the local field population yields a very low probability that this star is a chance-aligned field RRL, making Trumpler 5 the most compelling Milky Way case of an RRL associated with an intermediate-age open cluster to date.
I will briefly place this result in the context of recent population-level searches for RRL stars in intermediate-age clusters in the Magellanic Clouds, which provide independent statistical support for a low incidence of RRL formation at intermediate ages. Finally, I will discuss the implications of the Trumpler 5 RRL for models of horizontal-branch evolution and possible non-canonical formation channels, as well as the prospects for spectroscopic follow-up to further constrain its nature.