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1. Metallicity of Galactic RR Lyrae from Optical and Infrared Light Curves. II. Period–Fourier–Metallicity Relations for First Overtone RR Lyrae

   https://doi.org/10.3847/1538-4357/ac67ee  

   Mullen, Joseph P.; Marengo, Massimo; Martínez-Vázquez, Clara E.; Bono, Giuseppe; Braga, Vittorio F.; Chaboyer, Brian; Crestani, Juliana; Dall’Ora, Massimo; Fabrizio, Michele; Fiorentino, Giuliana; et al (June 2022, The Astrophysical Journal)

   Abstract We present new period-ϕ₃₁-[Fe/H] relations for first-overtone RRL stars (RRc), calibrated over a broad range of metallicities (−2.5 ≲ [Fe/H] ≲ 0.0) using the largest currently available set of Galactic halo field RRL with homogeneous spectroscopic metallicities. Our relations are defined in the optical (ASAS-SNVband) and, inaugurally, in the infrared (WISEW1andW2bands). OurV-band relation can reproduce individual RRc spectroscopic metallicities with a dispersion of 0.30 dex over the entire metallicity range of our calibrator sample (an rms smaller than what we found for other relations in literature including nonlinear terms). Our infrared relation has a similar dispersion in the low- and intermediate-metallicity range ([Fe/H] ≲ −0.5), but tends to underestimate the [Fe/H] abundance around solar metallicity. We tested our relations by measuring both the metallicity of the Sculptor dSph and a sample of Galactic globular clusters, rich in both RRc and RRab stars. The average metallicity we obtain for the combined RRL sample in each cluster is within ±0.08 dex of their spectroscopic metallicities. The infrared and optical relations presented in this work will enable deriving reliable photometric RRL metallicities in conditions where spectroscopic measurements are not feasible; e.g., in distant galaxies or reddened regions (observed with upcoming Extremely Large Telescopes and the James Webb Space Telescope), or in the large sample of new RRL that will be discovered in large-area time-domain photometric surveys (such as the LSST and the Roman space telescope). 

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2. Photometric Metallicity and Distance Estimates for ∼70,000 RR Lyrae Stars from the Zwicky Transient Facility

   https://doi.org/10.3847/1538-4365/adbcad  

   He, Shun-Xuan; Huang, Yang; Li, Xin-Yi; Zhang, Hua-Wei; Liu, Gao-Chao; Beers, Timothy C; Wu, Hong; Fan, Zhou (April 2025, The Astrophysical Journal Supplement Series)

   Abstract Utilizing Zwicky Transient Facility (ZTF) data and existing RR Lyrae stars (RRLs) catalogs, this study achieves the first calibration of theP−ϕ₃₁−R₂₁− [Fe/H] andP−ϕ₃₁−A₂−A₁− [Fe/H] relations in the ZTF photometric system for RRab and RRc stars. We also recalibrate the period–absolute magnitude–metallicity (PMZ) and period–Wesenheit–metallicity (PWZ) relations in the ZTFgribands for RRab and RRc stars. Based on nearly 4100 stars with precise measurements ofP,ϕ₃₁,A₂, andA₁, and available spectroscopic metallicity estimates, the photometric metallicity relations exhibit strong internal consistency across different bands, supporting the use of a weighted averaging method for the final estimates. The photometric metallicity estimates of globular clusters based on RR Lyrae members also show excellent agreement with high-resolution spectroscopic measurements, with a typical scatter of 0.15 dex for RRab stars and 0.14 dex for RRc stars, respectively. Using hundreds of local RRLs with newly derived photometric metallicities and precise Gaia Data Release 3 parallaxes, we establish the PMZ and PWZ relations in multiple bands. Validation with globular cluster RR Lyrae members reveals typical distance errors of 3.1% and 3.0% for the PMZ relations, and 3.1% and 2.6% for the PWZ relations for RRab and RRc stars, respectively. Compared to PMZ relations, the PWZ relations are tighter and almost unbiased, making them the recommended choice for distance calculations. We present a catalog of 73,795 RRLs with precise photometric metallicities; over 95% of them have accurate distance measurements. Compared to Gaia DR3, approximately 25,000 RRLs have precise photometric metallicities and distances derived for the first time. 

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3. Standard Galactic field RR Lyrae II: a Gaia DR2 calibration of the period–Wesenheit–metallicity relation

   https://doi.org/10.1093/mnras/stz2814  

   Neeley, Jillian R.; Marengo, Massimo; Freedman, Wendy L.; Madore, Barry F.; Beaton, Rachael L.; Hatt, Dylan; Hoyt, Taylor; Monson, Andrew J.; Rich, Jeffrey A.; Sarajedini, Ata; et al (October 2019, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT RR Lyrae stars have long been popular standard candles, but significant advances in methodology and technology have been made in recent years to increase their precision as distance indicators. We present multiwavelength (optical UBVRcIc and Gaia G, BP, RP; near-infrared JHKs; mid-infrared [3.6], [4.5]) period–luminosity–metallicity (PLZ), period–Wesenheit–metallicity (PWZ) relations, calibrated using photometry obtained from the Carnegie RR Lyrae Program and parallaxes from the Gaia second data release for 55 Galactic field RR Lyrae stars. The metallicity slope, which has long been predicted by theoretical relations, can now be measured in all passbands. The scatter in the PLZ relations is on the order of 0.2 mag, and is still dominated by uncertainties in the parallaxes. As a consistency check of our PLZ relations, we also measure the distance modulus to the globular cluster M4, the Large Magellanic Cloud and the Small Magellanic Cloud, and our results are in excellent agreement with estimates from previous studies. 

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4. Probing the Galactic halo with RR Lyrae stars – IV. On the Oosterhoff dichotomy of RR Lyrae stars

   https://doi.org/10.1093/mnras/stad2681  

   Zhang, Shan; Liu, Gaochao; Huang, Yang; Lv, Zongfei; Bird, Sarah_Ann; Chen, Bingqiu; Zhang, Huawei; Beers, Timothy_C; Li, Xinyi; Tian, Haijun; et al (September 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We use 3653 (2661 RRab, 992 RRc) RR Lyrae stars (RRLs) with 7D (3D position, 3D velocity, and metallicity) information selected from Sloan Digital Sky Survey, Large Sky Area Multi-Object Fiber Spectroscopic Telescope, and Gaia EDR3, and divide the sample into two Oosterhoff groups (Oo I and Oo II) according to their amplitude–period behaviour in the Bailey diagram. We present a comparative study of these two groups based on chemistry, kinematics, and dynamics. We find that Oo I RRLs are relatively more metal-rich, with predominately radially dominated orbits and large eccentricities, while Oo II RRLs are relatively more metal-poor, and have mildly radially dominated orbits. The Oosterhoff dichotomy of the Milky Way’s halo is more apparent for the inner-halo region than for the outer-halo region. Additionally, we also search for this phenomenon in the haloes of the two largest satellite galaxies, the Large and Small Magellanic clouds, and compare over different bins in metallicity. We find that the Oosterhoff dichotomy is not immutable, and varies based on position in the Galaxy and from galaxy to galaxy. We conclude that the Oosterhoff dichotomy is the result of a combination of stellar and galactic evolution, and that it is much more complex than the dichotomy originally identified in Galactic globular clusters. 

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5. On the Use of Field RR Lyrae as Galactic Probes. II. A New ΔS Calibration to Estimate Their Metallicity*

   https://doi.org/10.3847/1538-4357/abd183  

   Crestani, J.; Fabrizio, M.; Braga, V. F.; Sneden, C.; Preston, G.; Ferraro, I.; Iannicola, G.; Bono, G.; Alves-Brito, A.; Nonino, M.; et al (February 2021, The Astrophysical Journal)

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