RR Lyrae variables (RRLs) are well-studied pulsating stars and generally accepted as old stars (> 10 Gyr). Most RRLs are in addition metal-poor, but metal-rich ([Fe/H]>−1) RRLs have been found in the solar neighborhood and the Galactic bulge. However, standard theory of stellar evolution does not predict such metal-rich RRLs. Recent papers suggested that they are younger objects (maybe ∼1 Gyr or even younger), evolved through mass exchange within binary systems. This scenario would weaken the assumption that RRLs are all old, having a broad impact on stellar and galactic astronomy. An alternative explanation is that they are isolated stars but mass loss during the RGB phase is enhanced. To discriminate these scenarios, detailed chemical abundances would provide crucial information on the metal-rich RRLs, although measurements of the detailed abundances of RRLs have been scarce and even suggested to be inaccurate. In this program, we collect IRD near-IR spectra of the metal-rich RRLs to measure their abundances of a few elements and distinguish the two scenarios. If [α/Fe] is found to be low as expected for thin-disk stars (∼8 Gyr or younger), the binary evolution scenario would be supported. In contrast, high [α/Fe] would suggest that the RRLs are old.
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