Low Mass X-ray Binaries and Metallicity Dependence: Story of Failures

Observations of galactic and extra-galactic globular clusters have shown that on average metal-rich clusters are ∼ 3 times as likely to contain a bright X-ray source than their metal-poor counterparts. We propose that this can be explained by taking into account the difference in the stellar structure of main sequence donors with masses between ∼ 0.85M⊙ and ∼ 1.25M⊙ at different metallicities. Metal-poor main sequence stars in this mass range do not have an outer convective zone while metal-rich stars do. The absence of this zone turns off magnetic braking, a powerful mechanism of orbital shrinkage, leading to the failure of dynamically formed main sequence neutron star binaries to start mass transfer or appear as bright low-mass X-ray binaries. Subject headings: binaries: close — stars: magnetic fields — X-rays: binaries — galaxies: star clusters — globular clusters: general — stellar dynamics