Metallicity of Galactic RR Lyrae from Optical and Infrared Light Curves. I. Period-Fourier-Metallicity Relations for Fundamental-mode RR Lyrae

Mullen, Joseph P.; Marengo, Massimo; Martínez-Vázquez, Clara E.; Neeley, Jillian R.; Bono, Giuseppe; Dall'Ora, Massimo; Chaboyer, Brian; Thévenin, Frédéric; Braga, Vittorio F.; Crestani, Juliana; Fabrizio, Michele; Fiorentino, Giuliana; Gilligan, Christina K.; Monelli, Matteo; Stetson, Peter B.
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The Astrophysical Journal

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We present newly calibrated period-φ31-[Fe/H] relations for fundamental-mode RR Lyrae stars in the optical and, for the first time, mid-infrared. This work's calibration data set provides the largest and most comprehensive span of parameter space to date, with homogeneous metallicities from -3 ≲ [Fe/H] ≲ 0.4 and accurate Fourier parameters derived from 1980 ASAS-SN (V band) and 1083 WISE (NEOWISE extension, W1 and W2 bands) RR Lyrae stars with well-sampled light curves. We compare our optical period-φ31-[Fe/H] relation with those available in the literature and demonstrate that our relation minimizes systematic trends in the lower and higher metallicity range. Moreover, a direct comparison shows that our optical photometric metallicities are consistent with both those from high-resolution spectroscopy and globular clusters, supporting the good performance of our relation. We found an intrinsic scatter in the photometric metallicities (0.41 dex in the V band and 0.50 dex in the infrared) by utilizing large calibration data sets covering a broad metallicity range. This scatter becomes smaller when optical and infrared bands are used together (0.37 dex). Overall, the relations derived in this work have many potential applications, including large-area photometric surveys with James Webb Space Telescope in the infrared and LSST in the optical.
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