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

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 μ 0 = 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.

Microlensing Constraints on the Mass of Single Stars from HST Astrometric Measurements*

“…Finally, we used the method of Bramich & Freudling (2012) to derive photometric offsets to be applied to each epoch, in order to correct for errors in fitted values of the photometric scale factor. This step was shown by Kains et al (2015) to lead to significant improvements in light curve photometric scatter. Further details of the algorithms used for each step can be found in Bramich (2008).…”

New variable stars towards the Galactic Bulge. I. The bright regime★†

“…We carried out searches for variable stars by using a number of independent methods. We first used the variability index S R , as defined by Kains et al (2013aKains et al ( , 2015; briefly, this quantifies the improvement in the phased light curve using the best-fit period compared to a random period. This is done by measuring the respective string lengths of the phased light curves, i.e.…”

Variable stars in the bulge globular cluster NGC 6401

“…Although it is not possible to suppress them completely, it is often the case that substantial qualitative improvements can be made when dealing with time-series photometry (e.g. ; Kains et al (2015)). We used the method from Bramich & Freudling (2012) to determine the magnitude offsets Z k that should be applied to each photometric measurement from the image k, which translates into a correction for any systematic errors introduced into the DIA photometry due to errors in the fitted photometric scale factors p(t).…”

“…For the RRab stars, we used Kovacs & Walker (2001) to derive the absolute magnitude MV = − 1.876 log P − 1.158 A1 + 0.821 A3 + K0, (14) where the zero-point of the calibration is K0 = 0.41 ± 0.02 mag, as discussed in Kains et al (2015).…”

“…This paper belongs to a series of studies of variable stars in globular clusters (e.g. Arellano Ferro et al (2015); Kains et al (2015); Arellano ; Figuera Jaimes et al (2013); Bramich et al (2011)) aimed primarily at Fourier decomposing the light curves of RR Lyrae stars in search of physical parameters and at updating the variability census using modern instruments and photometric reduction methods. In particular, the combination of modern CCDs and the technique of difference image analysis (Alard & Lupton 1998;Bramich 2008) allow us to probe for variability closer to the crowded central regions of the clusters than was previously possible.…”