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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. 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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3. 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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4. 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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5. Milky Way archaeology using RR Lyrae and type II Cepheids: I. The Orphan stream in 7D using RR Lyrae stars

   https://doi.org/10.1051/0004-6361/202140422  

   Prudil, Z.; Hanke, M.; Lemasle, B.; Crestani, J.; Braga, V. F.; Fabrizio, M.; Koch-Hansen, A. J.; Bono, G.; Grebel, E. K.; Matsunaga, N.; et al (April 2021, Astronomy & Astrophysics)

   null (Ed.)

   We present a chemo-dynamical study of the Orphan stellar stream using a catalog of RR Lyrae pulsating variable stars for which photometric, astrometric, and spectroscopic data are available. Employing low-resolution spectra from the Sloan Digital Sky Survey (SDSS), we determined line-of-sight velocities for individual exposures and derived the systemic velocities of the RR Lyrae stars. In combination with the stars’ spectroscopic metallicities and Gaia EDR3 astrometry, we investigated the northern part of the Orphan stream. In our probabilistic approach, we found 20 single mode RR Lyrae variables likely associated with the Orphan stream based on their positions, proper motions, and distances. The acquired sample permitted us to expand our search to nonvariable stars in the SDSS dataset, utilizing line-of-sight velocities determined by the SDSS. We found 54 additional nonvariable stars linked to the Orphan stream. The metallicity distribution for the identified red giant branch stars and blue horizontal branch stars is, on average, −2.13 ± 0.05 dex and −1.87 ± 0.14 dex, with dispersions of 0.23 and 0.43 dex, respectively. The metallicity distribution of the RR Lyrae variables peaks at −1.80 ± 0.06 dex and a dispersion of 0.25 dex. Using the collected stellar sample, we investigated a possible link between the ultra-faint dwarf galaxy Grus II and the Orphan stream. Based on their kinematics, we found that both the stream RR Lyrae and Grus II are on a prograde orbit with similar orbital properties, although the large uncertainties on the dynamical properties render an unambiguous claim of connection difficult. At the same time, the chemical analysis strongly weakens the connection between both. We argue that Grus II in combination with the Orphan stream would have to exhibit a strong inverse metallicity gradient, which to date has not been detected in any Local Group system. 

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