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

TL;DR

This study conducted a detailed search for variable stars in NGC 288, refined period measurements of known variables, and estimated the cluster's metallicity and distance using multiple methods, finding consistent results across different variable types.

Contribution

It provides new measurements of metallicity and distance for NGC 288, combining CCD photometry with literature data, and identifies double mode pulsators among SX Phe stars.

Findings

No new variables detected above threshold limits.

Refined periods for all known variables.

Consistent distance estimates around 8.9 kpc.

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 $V\approx19$ mag, was performed using difference image analysis (DIA). For stars of $\approx15$ mag, measurement accuracies of $\approx8$ mmag and $\approx10$ 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 ${\rm [Fe/H]}_{ZW}=-1.62\pm0.02$ (statistical) $\pm0.14$ (systematic). A true distance modulus of $14.768\pm0.003$ mag (statistical) $\pm0.042$ mag (systematic), or a distance of $8.99\pm0.01$ kpc (statistical) $\pm0.17$ 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 $8.9\pm0.3$ kpc. The SX Phe stars V5 and V9 are found to be double mode pulsators.