A Census of Variability in Globular Cluster M68 (NGC 4590)

Abstract

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]=$-2.07\pm 0.06$ on the ZW scale, and 4 independent estimates of the distance modulus (${\mu }_0\sim$ 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.

arXiv comment25 pages, 18 figures, A&A in press

References in corpus (59)

• Final Results from the Hubble Space Telescope Key Project to Measure the Hubble Constant
• The Dartmouth Stellar Evolution Database
• Na-O Anticorrelation and HB. VII. The chemical composition of first and second-generation stars in 15 globular clusters from GIRAFFE spectra
• Intrinsic iron spread and a new metallicity scale for Globular Clusters
• Na-O Anticorrelation and HB. VIII. Proton-capture elements and metallicities in 17 globular clusters from UVES spectra
• Homogeneous Photometry for Star Clusters and Resolved Galaxies. II. Photometric Standard Stars
• The Ages of 55 Globular Clusters as Determined Using an Improved delta V_TO^HB Method Along with Color-Magnitude Diagram Constraints, and Their Implications for Broader Issues
• Variable Stars in Galactic Globular Clusters
• The Third US Naval Observatory CCD Astrograph Catalog (UCAC3)
• Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
• The ACS Survey of Galactic Globular Clusters. II. Stellar Evolution Tracks, Isochrones, Luminosity Functions, and Synthetic Horizontal-Branch Models
• Ages of Globular Clusters from Hipparcos Parallaxes of Local Subdwarfs
• Homogeneous age dating of 55 Galactic globular clusters. Clues to the Galaxy formation mechanisms
• Difference imaging photometry of blended gravitational microlensing events with a numerical kernel
• Galactic Globular Cluster Relative Ages
• A New Algorithm For Difference Image Analysis
• A multi-color and Fourier study of RR Lyrae variables in the globular cluster NGC 5272 (M3)
• Astrometry and photometry with HST WFC3. II. Improved geometric-distortion corrections for 10 filters of the UVIS channel
• Galactic Globular Cluster Metallicity Scale from the Ca II Triplet. I. Catalog
• A pulsational approach to near infrared and visual magnitudes of RR Lyrae stars
• Astrometry with Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator RR Lyrae
• Metal Abundances, Radial Velocities and other Physical Characteristics for the RR Lyrae Stars in the Kepler Field
• A Study of the B-V Colour Temperature Relation
• Metal abundances of RR Lyrae stars in the bar of the Large Magellanic Cloud
• Towards the absolute planes: a new calibration of the Bolometric Corrections and Temperature scales for Population II Giants
• Difference Image Analysis: Extension to a Spatially Varying Photometric Scale Factor and Other Considerations
• Variable Stars in the Unusual, Metal-Rich Globular Cluster NGC 6388
• Ages of globular clusters: a new approach
• Variable Stars in the Unusual, Metal-Rich Globular Cluster NGC 6441
• The RR Lyrae variables and Horizontal Branch of NGC6656 (M22)
• [Fe/H] relations for c-type RR Lyrae Variables based upon Fourier Coefficients
• Empirical relations for cluster RR Lyrae stars revisited
• Testing Newtonian Gravity with AAOmega: Mass-to-Light Profiles of Four Globular Clusters
• BVI time series data of the Galactic Globular Cluster NGC3201. I. RR Lyrae stars
• Exploring the variable stars in the globular cluster NGC5024 (M53): New RR Lyrae and SX Phoenicis stars
• CCD time-series photometry of the globular cluster NGC 6981: Variable star census and physical parameter estimates
• The Globular Cluster NGC 5286. II. Variable Stars
• Systematic Trends In Sloan Digital Sky Survey Photometric Data
• An Aligned Stream of Low-Metallicity Clusters in the Halo of the Milky Way
• Long-term photometric monitoring of RR Lyr stars in M3
• Variable stars in the Globular Cluster NGC 2808
• SX Phoenicis Stars in the Globular Cluster NGC 5466
• Linear nonadiabatic properties of SX Phoenicis variables
• Metallicities for Double Mode RR Lyrae in the Large Magellanic Cloud
• CCD time-series photometry of the globular cluster NGC 5053: RR Lyrae, Blue Stragglers and SX Phoenicis stars revisited
• High Frame-rate Imaging Based Photometry, Photometric Reduction of Data from Electron-multiplying Charge Coupled Devices (EMCCDs)
• Variable stars in the globular cluster NGC 7492. New discoveries and physical parameters determination
• New SX Phoenicis Stars in the Globular Cluster M53
• A detailed census of variable stars in the globular cluster NGC 6333 (M9) from CCD differential photometry
• The Determination Of Reddening From Intrinsic VR Colors Of RR Lyrae Stars
• CoRoT light curves of RR Lyrae stars. CoRoT 101128793: long-term changes in the Blazhko effect and excitation of additional modes
• Estimating the parameters of globular cluster M 30 (NGC 7099) from time-series photometry
• Age and Metallicity Estimation of Globular Clusters from Stromgren Photometry
• Variable stars in the bar of the Large Magellanic Cloud: the photometric catalogue
• Constraining the parameters of globular cluster NGC 1904 from its variable star population
• CCD Photometry of the globular cluster NGC 5466. RR Lyrae light curve decomposition and the distance scale
• EMCCD photometry reveals two new variable stars in the crowded central region of the globular cluster NGC 6981
• A stellar population synthesis approach to the Oosterhoff dichotomy
• Fourier Decomposition of RR Lyrae light curves and the SX Phe population in the central region of NGC 3201

Cited by in corpus (18)

• New LZ and PW(Z) relations of RR Lyrae stars calibrated with Gaia EDR3 parallaxes
• Microlensing constraints on the mass of single stars from HST astrometric measurements
• RR Lyrae stars and the horizontal branch of NGC 5904 (M5)
• Revisiting the variable star population in NGC~6229 and the structure of the Horizontal Branch
• Difference image analysis: The interplay between the photometric scale factor and systematic photometric errors
• The use of double-mode RR Lyrae stars as robust distance and metallicity indicators
• On the Use of Field RR Lyrae as Galactic Probes. VI. Mixed mode RR Lyrae variables in Fornax and in nearby dwarf galaxies
• Red noise versus planetary interpretations in the microlensing event OGLE-2013-BLG-446
• The Globular Cluster NGC 6402 (M14). II. Variable Stars
• A New Overview of Secular Period Changes of RR Lyrae stars inM5
• A vindication of the RR Lyrae Fourier light curve decomposition for the calculation of metallicity and distance in globular clusters
• Variable stars in the bulge globular cluster NGC 6401
• Lithium in the lower red giant branch of 5 Galactic globular clusters
• Pulsations of the SX Phe Star BL Camelopardalis
• On the photometric metallicity of Oosterhoff-type I and II RRab stars
• New variable stars in the Galactic Bulge: I. The bright regime
• Study of UV bright sources in globular cluster NGC 4590 using Ultraviolet Imaging Telescope (UVIT) observations
• RR Lyrae mode switching in globular cluster M 68 (NGC 4590)