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

Abstract

RR Lyrae stars have long been considered unequivocal tracers of old (>10 Gyr) and metal-poor ($\mathrm{[Fe/H]}<-0.5$) stellar populations. First, because these populations are where they are readily found and because, according to canonical stellar evolution models for isolated stars, these are the only populations where RR Lyrae should exist. Recent independent results, however, are challenging this view and pointing at the existence of intermediate-age RR Lyrae, only a few (2$-$5) Gyrs old. Our goal in this work is to provide direct evidence of the existence of intermediate-age RR Lyrae by searching for these stars in Milky Way open clusters, where the age association will be direct and robust. We searched a catalogue of over 3,000 open clusters with published kinematically associated member stars by crossmatching it against a compilation of the largest publicly available RR Lyrae surveys (Gaia, ASAS-SN, PanStarrs1, Zwicky Transient Facility and OGLE-IV). We identified one star as a bona fide RR Lyrae variable and robust member of the 2$-$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 ($0.049\pm 0.013$%) that the star is a background field RR Lyrae and provide initial constraints on a possible binary companion based on its position in the colour-absolute magnitude diagram. Currently a source of debate, the Trumpler 5 RR Lyrae provides the most direct evidence to date of the existence of RR Lyrae stars at much younger ages than traditionally expected and adds to the mounting evidence supporting their existence.

Figures (9)

• Figure 1: Phase-folded light curves (top row) and location of the Trumpler 5 RRL in the Period-Amplitude diagram (bottom row) for the four surveys with publicly available time series data, from left to right: GaiaClementini2023_SOS_GaiaDR3, PS1 Sesar2017c, ASAS-SN Jayasinghe2019a and OGLE-IV Soszynski2016. In the bottom row the location of the Trumpler 5 RRL (star symbol) is shown in comparison to RRLs from each survey in the selected band. See text for details. Error bars for the magnitudes are plotted in all panels in the top row but are smaller than the symbol size in all cases except for ASAS-SN.
• Figure 2: The Trumpler 5 RRL (star) and cluster members (light gray) are shown, from left to right, in the plane of the sky (DEC versus RA), proper motions (pmDEC versus pmRA), parallax and Gaia CMD. In the first panel, $r_t$ and $r_{50}$ are shown with the dotted and dashed lines, these correspond respectively to the tidal radius and radius enclosing 50% of the members according to Hunt2023. In the fourth panel, the dotted lines correspond to instability strip (IS) limits from Marconi2015 for the cluster's metallicity; the dashed line shows the expected apparent G-band magnitude for the HB (from the empirical PLZ relation) at the cluster's metallicity Donati2015, and the star's parallax distance and extinction according to Green2019; the shading represents the uncertainty in the HB luminosity corresponding to the full range of (observed) RRL metallicities $[\mathrm{Fe/H}] \in [0.,2.0]$ dex and the uncertainties in the apparent magnitude and colour of the RRL are smaller than the star symbol. The agreement with the RRL's apparent magnitude is remarkable and the star lies right at the edge of the limits of the IS (see discussion in Sec. \ref{['s:camd']}).
• Figure 3: Location of the Trumpler 5 RRL (black star) with respect to the field population in the proper motions (top panel) and parallax versus pmRA (bottom panel) planes. Iso-contours represent the density of cluster members (orange), all field stars (gray) within the cluster's tidal radius, and 2000 field RRL stars (violet) within in a FOV of 25$^\circ$ around the cluster. A similar result is found when using proper motion in declination (not-shown). The analysis yields a final probability $(0.049 \pm 0.013)\%$ that the RRL is a chance interloper.
• Figure 4: CAMD for the cluster members (gray) and RRL (purple dots with error bars). Possible primary RRL stars are shown as coloured circles, and the corresponding secondary companions as triangles. The two tracks shown for primaries correspond to the G19 (large circles) and L22 extinctions (small circles). The colour scale represents the mass of the companion star. The IS limits for RRab from Marconi2015 and expected HB luminosity for the cluster's metallicity from the Garofalo2022 PLZ relation (as in Fig. \ref{['f:5d']}) are shown by the dotted lines, the shaded areas correspond to their respective uncertainties.
• Figure 5: Difference between the observed parallax and the parallax predicted from theoretical (filled circles) and empirical (open circles) PLZ relations for different photometric bands. The equivalent difference in magnitude is also represented in the right-hand axis. Results for the predicted parallax are shown for assumed extinctions from the G19 (black) and L22 (gray) dust maps. Since errors in this parallax difference are dominated by the uncertainty in the observed parallax, they are the same for all points and shown only for one band for guidance. Empirical PLZ relations used are from Narloch2024 for PS1 bands, from Prudil2024 for $I$, $BP$ and $RP$ and from Garofalo2022 for the $G$ and $W_{G,BP-RP}$ band. Theoretical relations used are from Marconi2021 for $W_{G,BP-RP}$, Marconi2015 for $I$ and Marconi2022 for the LSST $r$ and $i$ bands (assumed here to match the PS1 bands, based on filter transmission curves for both surveys).