The first X-ray study of NGC 2243, one of the most metal-poor Galactic open star clusters

We propose to observe one of the most metal-poor Galactic open clusters, NGC 2243, to make a census of its X-ray sources down to Lx=1e30 erg/s. The goal is to investigate the observed dichotomy in X-ray emissivity (0.3-7 keV) of old low-density star clusters: per unit mass, old open clusters are about 10x more luminous than globular clusters of similar mass and low central density. A fundamental difference between these cluster types is their metallicity, with open clusters typically being 10x more metal-rich than globular clusters. With [Fe/H]=-0.5 and an age of 4 Gyr, NGC 2243 is key to understanding the so far unstudied effect of metallicity on the X-ray emissivity of low-density clusters. Chandra's sharp PSF provides the sensitivity that is crucial for such a survey of faint sources.