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Ultra-Compact binaries in globular clusters

Ultra Compact (UC) binary systems have orbital periods, P_orb, shorter than ~70 min and are therefore at the short period end of the binary orbital period distribution. Those UC binaries with neutron star accretors are known as UC X-ray binaries and those with white dwarf accretors as AM CVn binaries (after their prototype). It is known that UC X-ray binaries are more abundant in globular clusters compared to that found in Galactic fields (eg van der Sluys, Verbunt \& Pols 2005 and references therein). On the other hand no AM CVn systems have been found in globular clusters, while 20 are known in the Galactic field (for a review of AM CVn systems see Nelemans 2005).

Recent predictions (Ivanova et al 2006) suggest that the same order of AM CVn systems should be present in globulars as hydrogen accreting binaries with white dwarf accretors (the `classical' cataclysmic variables, CVs). Since 10's of CVs have been found in globular clusters (22 alone in 47 Tuc, Edmonds et al 2003) the same order of AM CVn systems should be present in globular clusters. However, so far, none have been discovered.

One method of detecting AM CVn systems with P_orb <30 min is through their optical intensity variations (those systems with P_orb >30 min tend not to show evidence of the orbital period in their optical light curve). For a system with P_orb =20 min, the predicted absolute magnitude for a AM CVn is M_V ~7 (Bildsten et al 2006), which gives a V~18.5 for a distance of 2 kpc and V~20.5 for a distance of 5 kpc.

Globular clusters have been extensively studied in optical bands, but the exposure times and the sampling rates have generally precluded the detection of rapidly varying objects such as AM CVn systems. However, the strategy of the RApid Temporal Survey (RATS) is to obtain a series of short exposures (~30 sec) of the same field for 2--3 hrs. Objects which are found to be variable on periods shorter than 70 min are then subject to followup spectroscopic and photometric observations to identify their nature.

In addition to our goal of identifying ultra-compact binaries in the Galactic field, we have a project to discover ultra-compact binaries in globular clusters. Pilot observations took place using the ESO 2.2m telescope and the wide field imager and we observed M4, M22 and NGC 6540. A paper detailing our results is in preparation.

Further observations of globular clusters are taking place using SALT, of which Armagh Observatory is a member of the UK consortium. Although the image quality of SALT is not perfect at this stage recent improvements have very recently been announced.

An image of the core of M12 taken using a 30 sec exposure in the B filter using SALT is shown below.