We present new theoretical period-luminosity-metallicity (PLZ) relations for RR Lyrae stars (RRLs) at Spitzer and WISE wavelengths. The PLZ relations were derived using nonlinear, time-dependent convective hydrodynamical models for a broad range of metal abundances (Z=0.0001-0.0198). In deriving the light curves, we tested two sets of atmospheric models and found no significant difference between the resulting mean magnitudes. We also compare our theoretical relations to empirical relations derived from RRLs in both the field and in the globular cluster M4. Our theoretical PLZ relations were combined with multi-wavelength observations to simultaneously fit the distance modulus, μ 0 , and extinction, A V , of both the individual Galactic RRL and of the cluster M4. The results for the Galactic RRL are consistent with trigonometric parallax measurements from Gaia's first data release. For M4, we find a distance modulus of μ 0 =11.257±0.035 mag with A V =1.45±0.12 mag, which is consistent with measurements from other distance indicators. This analysis has shown that, when considering a sample covering a range of iron abundances, the metallicity spread introduces a dispersion in the PL relation on the order of 0.13 mag. However, if this metallicity component is accounted for in a PLZ relation, the dispersion is reduced to ∼0.02 mag at mid-infrared wavelengths.

We present a new complete Near-Infrared (NIR, JHK s ) census of RR Lyrae stars (RRLs) in the globular ω Cen (NGC 5139). We collected 15,472 JHK s images with 4-8m class telescopes over 15 years (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015) covering a sky area around the cluster center of 60×34 arcmin 2 . These images provided calibrated photometry for 182 out of the 198 cluster RRL candidates with ten to sixty measurements per band. We also provide new homogeneous estimates of the photometric amplitude for 180 (J), 176 (H) and 174 (K s ) RRLs. These data were supplemented with single-epoch JK s magnitudes from VHS and with single-epoch H magnitudes from 2MASS. Using proprietary optical and NIR data together with new optical light curves (ASAS-SN) we also updated pulsation periods for 59 candidate RRLs. As a whole, we provide JHK s magnitudes for 90 RRab (fundamentals), 103 RRc (first overtones) and one RRd (mixed-mode pulsator). We found that NIR/optical photometric amplitude ratios increase when moving from first overtone to fundamental and to long-period (P>0.7 days) fundamental RRLs. Using predicted Period-Luminosity-Metallicity relations, we derive a true distance modulus of 13.674±0.008±0.038 mag (statistical error and standard deviation of the median)-based a This publication makes use of data gathered with the Magellan/Baade Telescope at Las Campanas Observatory, the Blanco Telescope at Cerro Tololo Inter-American Observatory, NTT at La Silla (ESO Program IDs: 64.N-0038(A), 66.D-0557(A), 68.D-0545(A), 073.D-0313(A), ID 073.D-0313(A) and 59.A-9004(D)), VISTA at Paranal (ESO Program ID: 179.A-2010) and VLT at Paranal (ESO Program ID: ID96406).RRc RRab Global DM err a σ b DM err a σ b DM err a σ b mag mag mag mag mag mag mag mag mag -[Fe/H] from Sollima et al. (2006a)--Theoretical calibration-J 13.658 0.009 0.047 13.656 0.012 0.029 13.655 0.009 0.035 H 13.675 0.006 0.044 13.676 0.008 0.034 13.674 0.006 0.033 K s 13.697 0.005 0.041 13.687 0.007 0.026 13.690 0.005 0.029 mean JHKs 13.677 0.007 0.047 13.673 0.009 0.032 13.673 0.007 0.036 Overall mean: 13.674±0.008±0.038 -Empirical calibration-J 13.693 0.016 0.027 13.809 0.017 0.062 13.755 0.011 0.031 H 13.666 0.015 0.032 13.791 0.017 0.032 13.766 0.010 0.031 K s 13.707 0.015 0.034 13.819 0.017 0.025 13.772 0.010 0.029 mean JHKs 13.689 0.015 0.035 13.806 0.017 0.045 13.764 0.010 0.031 Overall mean: 13.757±0.014±0.056 -[Fe/H] from Braga et al. (2016)--Theoretical calibration-J 13.681 0.005 0.045 13.677 0.006 0.036 13.675 0.004 0.048 H 13.694 0.004 0.041 13.706 0.005 0.041 13.699 0.003 0.045 K s 13.723 0.003 0.045 13.713 0.005 0.037 13.720 0.003 0.051 mean JHKs 13.699 0.004 0.047 13.699 0.005 0.041 13.698 0.004 0.052 Overall mean: 13.698±0.004±0.048 -Empirical calibration-J 13.725 0.009 0.031 13.853 0.012 0.034 13.767 0.006 0.036 H 13.691 0.008 0.030 13.819 0.011 0.036 13.772 0.005 0.037 K s 13.742 0.008 0.032 13.839 0.011 0.037 13.791 0.005 0.039 mean JHKs 13.719 0.008 0.037 13.837 0.012 0.038 13.777 0.005 0.039Ove...

We present a detailed spectroscopic analysis of horizontal branch stars in the globular cluster NGC 3201. We collected optical (4580-5330Å), high resolution (∼34,000), high signal-to-noise ratio (∼200) spectra for eleven RR Lyrae stars and one red horizontal branch star with the multifiber spectrograph M2FS at the 6.5m Magellan telescope at the Las Campanas Observatory. From measured equivalent widths we derived atmospheric parameters and abundance ratios for α (Mg, Ca, Ti), iron peak (Sc, Cr, Ni, Zn) and s-process (Y) elements. We found that NGC 3201 is a homogeneous, monometallic ([Fe/H]=−1.47 ± 0.04), α-enhanced ([α/Fe]=0.37 ± 0.04) cluster. The relative abundances of the iron peak and s-process elements were found to be consistent with solar values. In comparison with other large stellar samples, NGC 3201 RR Lyraes have similar chemical enrichment histories as do those of other old (t≥10 Gyr) Halo components (globular clusters, red giants, blue and red horizontal branch stars, RR Lyraes). We also provided a new average radial velocity estimate for NGC 3201 by using a template velocity curve to overcome the limit of single epoch measurements of variable stars: V rad = 494 ± 2 km s −1 (σ=8 km s −1 ).