A census of variability in globular cluster M 68 (NGC 4590)^⋆

¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
e-mail: nkains@stsci.edu
² European Southern Observatory, Karl-Schwarzschild Straße 2, 85748 Garching bei München, Germany
³ Instituto de Astronomía, Universidad Nacional Autónoma de Mexico, 04310 Coyoácan, Mexico
⁴ SUPA School of Physics & Astronomy, University of St. Andrews, North Haugh, St. Andrews, KY16 9SS, UK
⁵ Qatar Environment and Energy Research Institute, Qatar Foundation, Tornado Tower, Floor 19, PO Box 5825, Doha, Qatar
⁶ Niels Bohr Institute, University of Copenhagen, Juliane Maries vej 30, 2100 Copenhagen, Denmark
⁷ Centre for Star and Planet Formation, Geological Museum, Øster Voldgade 5, 1350 Copenhagen, Denmark
⁸ Las Cumbres Observatory Global Telescope Network, 6740 Cortona Drive, Suite 102, Goleta, CA 93117, USA
⁹ School of Mathematical Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, UK
¹⁰ Qatar Foundation, PO Box 5825, Doha, Qatar
¹¹ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
¹² Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead, Wirral., CH41 1LD, UK
¹³ Dipartimento di Fisica “E.R. Caianiello”, Università degli Studi di Salerno, via Giovanni Paolo II 132, 84084 Fisciano, Italy
¹⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Italy
¹⁵ Istituto Internazionale per gli Alti Studi Scientifici (IIASS), via Giuseppe Pellegrino, 19, 84019 Vietri Sul Mare Salerno, Italy
¹⁶ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁷ Yunnan Observatories, Chinese Academy of Sciences, 650011 Kunming, PR China
¹⁸ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, 650011 Kunming, PR China
¹⁹ Korea Astronomy and Space Science Institute, 305–348 Daejeon, Korea
²⁰ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
²¹ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
²² Department of Physics, Sharif University of Technology, PO Box 11155–9161, Tehran, Iran
²³ Perimeter Institute for Theoretical Physics, 31 Caroline St. N., Waterloo ON, N2L 2Y5, Canada
²⁴ Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
²⁵ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, Sart Tilman, Bât. B5c, 4000 Liège, Belgium

Received: 14 July 2014
Accepted: 22 February 2015

Abstract

Aims. We analyse 20 nights of CCD observations in the V and I bands of the globular cluster M 68 (NGC 4590) and use them to detect variable objects. We also obtained electron-multiplying CCD (EMCCD) observations for this cluster in order to explore its core with unprecedented spatial resolution from the ground.

Methods. We reduced our data using difference image analysis to achieve the best possible photometry in the crowded field of the cluster. In doing so, we show that when dealing with identical networked telescopes, a reference image from any telescope may be used to reduce data from any other telescope, which facilitates the analysis significantly. We then used our light curves to estimate the properties of the RR Lyrae (RRL) stars in M 68 through Fourier decomposition and empirical relations. The variable star properties then allowed us to derive the cluster’s metallicity and distance.

Results. M 68 had 45 previously confirmed variables, including 42 RRL and 2 SX Phoenicis (SX Phe) stars. In this paper we determine new periods and search for new variables, especially in the core of the cluster where our method performs particularly well. We detect 4 additional SX Phe stars and confirm the variability of another star, bringing the total number of confirmed variable stars in this cluster to 50. We also used archival data stretching back to 1951 to derive period changes for some of the single-mode RRL stars, and analyse the significant number of double-mode RRL stars in M 68. Furthermore, we find evidence for double-mode pulsation in one of the SX Phe stars in this cluster. Using the different classes of variables, we derived values for the metallicity of the cluster of [Fe/H] = −2.07 ± 0.06 on the ZW scale, or −2.20 ± 0.10 on the UVES scale, and found true distance moduli μ₀ = 15.00 ± 0.11 mag (using RR0 stars), 15.00 ± 0.05 mag (using RR1 stars), 14.97 ± 0.11 mag (using SX Phe stars), and 15.00 ± 0.07 mag (using the M_V −[Fe/H] relation for RRL stars), corresponding to physical distances of 10.00 ± 0.49, 9.99 ± 0.21, 9.84 ± 0.50, and 10.00 ± 0.30 kpc, respectively. Thanks to the first use of difference image analysis on time-series observations of M 68, we are now confident that we have a complete census of the RRL stars in this cluster.