Variable Stars in the Globular Cluster NGC 288: [Fe/H] and Distance

Abstract

A search for variable stars in the globular cluster NGC 288 was carried out using a time-series of CCD images in the V and I filters. The photometry of all stellar sources in the field of view of our images, down to V≈19 mag, was performed using difference image analysis (DIA). For stars of ≈15 mag, measurement accuracies of ≈8 mmag and ≈10 mmag were achieved for V and I respectively. Three independent search strategies were applied to the 5525 light curves but no new variables were found above the threshold limits characteristic of our data set. The use of older data from the literature combined with the present data allowed the refinement of the periods of all known variables. Fourier light curve decomposition was performed for the RRab and the RRc stars to obtain an estimate of [Fe/H]ZW=-1.62±0.02 (statistical) ±0.14 (systematic). A true distance modulus of 14.768±0.003 mag (statistical) ±0.042 mag (systematic), or a distance of 8.99±0.01 kpc (statistical) ±0.17 kpc (systematic) was calculated from the RRab star. The RRc star predicts a discrepant distance about one kiloparsec shorter but it is possibly a Blazhko variable. An independent distance from the P-L relationship for SX Phe stars leads to a distance of 8.9±0.3 kpc.The SX Phe stars V5 and V9 are found to be double mode pulsators.