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Title: A Census of Variability in Globular Cluster M68 (NGC 4590)
Author: N. Kains, A. Arellano Ferro, R. Figuera Jaimes, D. M. Bramich, J. Skottfelt, U. G. Jørgensen, Y. Tsapras, R. A. Street, P. Browne, M. Dominik, K. Horne, M. Hundertmark, S. Ipatov, C. Snodgrass, I. A. Steele, K.A. Alsubai, V. Bozza, S. Calchi Novati, S. Ciceri, G. D'Ago, P. Galianni, S.-H. Gu, K. Harpsøe, T.C. Hinse, D. Juncher, H. Korhonen, L. Mancini, A. Popovas, M. Rabus, S. Rahvar, J. Southworth, J. Surdej, C. Vilela, X.-B. Wang, O. Wertz

We analyse 20 nights of CCD observations in the V and I bands of the globular cluster M68 (NGC 4590), using these 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.
We reduced our data using difference image analysis, in order to achieve the best possible photometry in the crowded field of the cluster. In doing so, we showed 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 M68 through Fourier decomposition and empirical relations. The variable star properties then allowed us to derive the cluster's metallicity and distance.
We determine new periods for the variable stars, and search for new variables, especially in the core of the cluster where our method performs particularly well. We detect an additional 4 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 in order to derive period changes for some of the single-mode RRL stars, and analyse the significant number of double-mode RRL stars in M68.
Furthermore, we find evidence for double-mode pulsation in one of the SX Phe stars in this cluster. Using the different types of variables, we derived an estimate of the metallicity, [Fe/H]=-2.07±0.06 on the ZW scale, and 4 independent estimates of the distance modulus (µ0~ 15.00 mag) for this cluster. Thanks to the first use of difference image analysis on time-series observations of M68, we are now confident that we have a complete census of the RRL stars in this cluster.

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Messier 68 (also M68, NGC 4590, Melotte 113 and GCl 20) is a magnitude +9.7 globular star cluster located 33,000 light-years away in the constellation Hydra. 

The cluster was discovered by French astronomer Charles-Joseph Messier on the 9th April 1780.

Right Ascension 12h 43m 36.4s, Declination +41° 01' 09"

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A ten billion-year stellar dance

M68_ssiotw30july12_L.jpg

The NASA/ESA Hubble Space Telescope offers this delightful view of the crowded stellar encampment called Messier 68, a spherical, star-filled region of space known as a globular cluster.
Mutual gravitational attraction among a cluster's hundreds of thousands or even millions of stars keeps stellar members in check, allowing globular clusters to hang together for many billions of years.
Astronomers can measure the ages of globular clusters by looking at the light of their constituent stars.

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