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T.J. Maccarone (University of Amsterdam), A. Kundu, S.E. Zepf (Michigan State University)
We will discuss implications of correlations between the X-ray and optical properties of globular clusters in early-type galaxies. The probability that a globular cluster contains an X-ray binary depends primarily on the cluster's mass and metallicity, with at most weak dependences on its age. The X-ray properties of the globular cluster X-ray sources also show a dependence on metallicity, with the metal poor clusters hosting sources with systematically harder X-ray spectra than the metal rich clusters.
Both metallicity effects can be explained in terms of an irradiation-induced wind model. The irradiation induced winds, unlike normal stellar winds, are expected to be stronger when driven from metal poor stars, since the two dominant channels for dissipation of the irradiating energy are line cooling and kinetic energy supplied to outflows. As a result, the metal poor X-ray binaries will see a larger fraction of their donor star's mass blown off a wind, rather than accreted onto the compact object, thereby reducing their accretion lifetime and hence their number density. The wind material will also absorb soft X-rays leading to the harder X-ray spectra seen from the metal poor clusters' X-ray binaries, and also explaining why the effect was seen most prominently in past ROSAT observations, and is not found in the Milky Way systems, which suffer from large interstellar absorption.
We will also briefly discuss the implications for the neutron star retention problem and the kick velocity of neutrons stars.
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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.