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Searching for variability in the globular cluster Messier 4 Ferdman, Robert Daniel

Abstract

Time-series data taken with the Hubble Space Telescope (HST) of a field six core radii (~ 5') from the centre of the globular cluster Messier 4 (M4) have been analysed in search of variable objects. These observations consisted of 98 x 1300 s exposures in the F606W (V) filter, and 148 x 1300 s exposures in the F814W (I) filter, covering a period of about 10 weeks in early 2001. Various selection criteria were employed to select candidate variable stars. These included: selection of outliers in the RMS and variability index vs. magnitude distributions, as well as selection of stars in the colourmagnitude diagram of the cluster that lie above the main sequence (MS) and between the MS and white dwarf sequence. White dwarf stars that lie within the empirical ZZ Ceti instability region were also selected as candidates. Period searches were then performed on the selected candidates using the phase dispersion minimization (PDM) technique. The reliability of the P DM search results for this dataset was tested using synthetic light curves of eclipsing binary star and sinusoidal light curves at a few different periods. Results from this analysis showed that there are probably no eclipsing binary stars or periodic variables with periods on the order of a half a day, down to limiting magnitudes of V ~ 25 and I ~ 24. This is consistent with the absence of contact binaries such as the W Ursae Majoris systems. However, one candidate variable star does show a increase in brightness of ~ 0.1 magnitudes in both bandpasses; this rise in amplitude seems to last for a few days. If this object is periodic, its period is too long to detect given the time sampling of the data. Possible explanations concerning the nature of this object include a binary system with a white dwarf primary and a low-mass main sequence secondary, or a B Y Draconis variable star. Both of these objects can exhibit nonperiodic flaring or low amplitude, periodic variability in the light curve due to starspots.

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