The Astronomical Journal, 152:170 (34pp), 2016 December doi:10.3847/0004-6256/152/6/170 © 2016. The American Astronomical Society. All rights reserved. ON THE RR LYRAE STARS IN GLOBULARS. IV. ω CENTAURI OPTICAL UBVRI PHOTOMETRY* V. F. Braga1,2,3, P. B. Stetson4, G. Bono1,3, M. Dall’Ora5, I. Ferraro3, G. Fiorentino6, L. M. Freyhammer7, G. Iannicola3, M. Marengo8, J. Neeley8, E. Valenti9, R. Buonanno1,10, A. Calamida11, M. Castellani3, R. da Silva2,3, S. Degl’Innocenti12,13, A. Di Cecco10, M. Fabrizio2,10, W. L. Freedman14, G. Giuffrida2,3, J. Lub15, B. F. Madore16, M. Marconi5, S. Marinoni2,3, N. Matsunaga17, M. Monelli18, S. E. Persson16, A. M. Piersimoni10, A. Pietrinferni10, P. Prada-Moroni12,13, L. Pulone3, R. Stellingwerf19, E. Tognelli12,13, and A. R. Walker20 1 Department of Physics, Università di Roma Tor Vergata, via della Ricerca Scientifica 1, I-00133 Roma, Italy 2 ASDC, via del Politecnico snc, I-00133 Roma, Italy 3 INAF-Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monte Porzio Catone, Italy 4 NRC-Herzberg, Dominion Astrophysical Observatory, 5071 West Saanich Road, Victoria BC V9E 2E7, Canada 5 INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, I-80131 Napoli, Italy 6 INAF-Osservatorio Astronomico di Bologna, Via Ranzani 1, I-40127 Bologna, Italy 7 Jeremiah Horrocks Institute of Astrophysics, University of Central Lancashire, Preston PR1 2HE, UK 8 Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA 9 European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei Munchen, Germany 10 INAF-Osservatorio Astronomico di Teramo, Via Mentore Maggini snc, Loc. Collurania, I-64100 Teramo, Italy 11 National Optical Astronomy Observatory, 950 N Cherry Avenue, Tucson, AZ 85719, USA 12 INFN, Sezione di Pisa, Largo Pontecorvo 3, I-56127, Pisa, Italy 13 Dipartimento di Fisica “Enrico Fermi,” Università di Pisa, Largo Pontecorvo 3, I-56127, Pisa, Italy 14 Department of Astronomy & Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA 15 Sterrewacht Leiden, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands 16 The Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101, USA 17 Kiso Observatory, Institute of Astronomy, School of Science, The University of Tokyo, 10762-30, Mitake, Kiso-machi, Kiso-gun, 3 Nagano 97-0101, Japan 18 Instituto de Astrofísica de Canarias, Calle Via Lactea s/n, E-38205 La Laguna, Tenerife, Spain 19 Stellingwerf Consulting, 11033 Mathis Mtn Rd SE, Huntsville, AL 35803, USA 20 Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, Casilla 603, La Serena, Chile Received 2016 July 21; revised 2016 September 13; accepted 2016 September 13; published 2016 November 21 ABSTRACT New accurate and homogeneous optical UBVRI photometry has been obtained for variable stars in the Galactic globular cluster ω Cen (NGC 5139). We secured 8202 CCD images covering a time interval of 24 years and a sky area of 84×48 arcmin. The current data were complemented with data available in the literature and provided new, homogeneous pulsation parameters (mean magnitudes, luminosity amplitudes, periods) for 187 candidate ω Cen RR Lyrae (RRLs). Among them we have 101RRc (first overtone) and 85RRab (fundamental) variables, and a single candidate RRd (double-mode) variable. Candidate Blazhko RRLs show periods and colors that are intermediate between the RRc and RRab variables, suggesting that they are transitional objects. A comparison of the period distribution and the Bailey diagram indicates that RRLs in ω Cen show a long-period tail not present in typical Oosterhoff II (OoII) globulars. The RRLs in dwarf spheroidals and in ultra-faint dwarfs have properties between Oosterhoff intermediate and OoII clusters. Metallicity plays a key role in shaping the above evidence. These findings do not support the hypothesis that ω Cen is the core remnant of a spoiled dwarf galaxy. Using optical period–Wesenheit relations that are reddening-free and minimally dependent on metallicity we find a mean distance to ω Cen of 13.71±0.08±0.01 mag (semi-empirical and theoretical calibrations). Finally, we invert the I-band period–luminosity–metallicity relation to estimate individual RRLs’ metal abundances. The metallicity distribution agrees quite well with spectroscopic and photometric metallicity estimates available in the literature. Key words: globular clusters: individual (omega Cen) – stars: distances – stars: horizontal-branch – stars: variables: RR Lyrae 1. INTRODUCTION Rix 2013) and was the first to show a clear and well defined ( The Galactic stellar system ω Cen lies at the crossroads of spread in metal abundance Norris & Da Costa 1995; Johnson ) α s r several open astrophysical problems. It is the most massive & Pilachowski 2010 in and in - and -process elements 6 (Johnson et al. 2009). On the basis of the above peculiarities it Milky Way globular cluster (GC)(4.05×10 Me[d/ 3 has also been suggested that ω Cen and a few other massive (5.5±0.2 kpc)] where d is the distance, D’Souza & Galactic globular clusters (GGCs) might have been the cores of pristine dwarf galaxies (Da Costa & Coleman 2008; Marconi * Based in part on proprietary data and on data obtained from the ESO Science et al. 2014). Archive Facility under multiple requests by the authors; and in part upon data The distance to ω Cen has been estimated using primary and distributed by the NOAO Science Archive. NOAO is operated by the Association of Universities for Research in Astronomy (AURA) under geometrical distance indicators. The tip of the red giant branch cooperative agreement with the National Science Foundation. This research (TRGB) was adopted by Bellazzini et al. (2004) and Bono et al. also benefited from the Digitized Sky Survey service provided by the Canadian (2008b) with distances ranging from 13.65 to 13.70 mag. The Astronomy Data Centre operated by the National Research Council of Canada – ( ) with the support of the Canadian Space Agency. A detailed description of the K-band period luminosity PL relations of RR Lyrae stars log of the observations used in this investigation is given in Table 1. (RRLs) have been adopted by Longmore et al. (1990), Sollima 1 The Astronomical Journal, 152:170 (34pp), 2016 December Braga et al. et al. (2006b), and Bono et al. (2008b). The distance moduli A detailed near-infrared (NIR) analysis was performed by they estimated range from 13.61 to 13.75 mag. On the other Del Principe et al. (2006) using time series data collected with hand, ω Cen distance moduli based on the relations between SOFI at NTT. They provided homogeneous JKs photometry for luminosity and iron abundance for RRLs range from 13.62 to 180 variables and provided a new estimate of the ω Cen 13.72 mag (Del Principe et al. 2006). The difference in distance distance modulus using the K-band PL relation between the different methods is mainly due to the intrinsic (13.77 ± 0.07 mag). A similar analysis was recently performed spread in the adopted diagnostics and in the reddening by Navarrete et al. (2015) based on a large set of images correction. collected with the VISTA telescope. They provided homo- Optical PL relations for SX Phoenicis stars were adopted by geneous JKs photometry for 189 probable member RRLs (101 McNamara (2011) who found a distance of 13.62±0.05 mag. RRc, 88 RRab) and discussed the pulsation properties of the One eclipsing variable has been studied by Kaluzny et al. entire sample in the NIR. In particular, they provided new NIR (2007), and they found a distance modulus of reference lines for Oosterhoff I (OoI) and Oosterhoff II (OoII) 13.49±0.14 mag and 13.51±0.12 mag for the two compo- clusters. Moreover, they further supported the evidence that nents. The key advantage in dealing with eclipsing binaries is RRab in ω Cen display properties similar to OoII systems. that they provide very accurate geometrical distances (Pietr- These investigations have been complemented with a detailed zyński et al. 2013). Estimates based on cluster proper motions optical investigation covering a sky area of more than 50 ( ) provide distance estimates that are systematically smaller than square degrees by Fernández-Trincado et al. 2015b . They ( ) obtained from the other most popular distance indicators detected 48 RRLs and the bulk of them 38 are located outside (13.27 mag, van Leeuwen et al. 2000; 13.31 ± 0.04 mag, the tidal radius. However, detailed simulations of the different Watkins et al. 2013). The reasons for this difference are not Galactic components and radial velocities for a sub-sample of yet clear. RRLs indicate a lack of tidal debris around the cluster. The modest distance and the large mass of ω Cen make this This is the fourth paper of a series focused on homogeneous ( ) stellar system a fundamental laboratory to constrain evolu- optical, NIR, and mid-infrared MIR photometry of cluster tionary and pulsation properties of old (t>10 Gyr) low-mass RRLs. The structure of the paper is as follows. In Section 2 we stars. The key advantage in dealing with stellar populations in present the optical multi-band UBVRI photometry that we this system is that they cover a broad range in metallicity collected for this experiment together with the approach (−2.0[Fe/H]−0.5, Pancino et al. 2002; −2.5[Fe/ adopted to perform the photometry on individual images and H]+0.5, Calamida et al. 2009; −2.2[Fe/H]−0.6, on the entire data set. In Section 3.1 we discuss in detail the identification of RRLs and the photometry we collected from Johnson & Pilachowski 2010) and they are located at the same the literature to provide homogeneous estimates of the RRL distance (Castellani et al.