Abstract
The primordial binary population is a key input parameter for any realistic model of dense star cluster dynamics but it remains poorly understood. Most of the close binaries in globular clusters are thought to be dominated by dynamically-formed systems. Theoretical calculations show that cataclysmic variables (CVs) can be formed directly from primordial binaries in or near the core of low core density globular clusters. We here report on the Chandra X-ray Observatory observations of the low core density globular cluster NGC 288. We detect four X-ray sources within the core radius and seven additional sources within the half-mass radius down to a limiting luminosity of Lx=7e30 erg/s (assuming cluster membership) in the 0.3-7 keV band. We also observed the cluster with the Hubble Space Telescope (HST) Advanced Camera for Surveys and identify optical counterparts to seven X-ray sources out of the nine sources within the HST field-of-view. Based on the X-ray and optical properties, we find 2-5 candidates of CVs or chromospherically active binaries, and 2-5 background galaxies inside the half-mass radius. Since the core density of NGC 288 is very low, the low-luminosity X-ray sources of NGC 288 found in the Chandra and HST observations is higher than the prediction on the basis of the collision frequency. We suggest that the CVs and chromospherically active binaries are primordial in origin, in agreement with theoretical expectation. Similar finding is also found in another low core density cluster M55 in which a CV is inside the core radius.