LOW-MASS X-RAY BINARIES IN GLOBULAR-CLUSTERS - A NEW METALLICITY EFFECT

Globular clusters (GCs) containing bright X-ray sources (L(x) > 10(36) ergs s(-1)), commonly associated with low-mass X-ray binaries (LMXBs), are found to be significantly denser and more metal-rich than normal nonX-ray clusters both in the Galaxy and in M31. Within a framework where LMXBs in GCs are generated via tidal captures in high-density clusters and (2 + 1) encounters in low-density globulars, the higher incidence of LMXBs with increasing metallicity is shown to be intrinsic and not just a by-product of other effects. Two possible mechanisms are examined: the first one assumes a dependence of the cluster IMF on metallicity as recently published in the literature. The number of observed LMXBs, more frequently occurring in metal-rich clusters, agrees with the predicted number of NS only if metallicity accounts for a minor contribution to the observed variation of the IMF slope. Other alternatives explored, such as the total variation of the observed IMF slopes is due to (1) just metallicity and (2) the combination of metallicity and position in the Galaxy lead to a clear-cut disagreement with the data. In turn, this result may indicate a flatter dependence of the cluster's IMF on metallicity than that deduced from observed cluster luminosity functions. The second mechanism assumes that, at fixed cluster density, the rate of tidar captures depends on radius and mass of the capturing star. Based on standard stellar models, stars with higher metal content have wider radii and higher masses, hence the rate of tidal captures increases with increasing metallicity. Moreover, since the fixed binary separation and masses of the two components, metal-rich stars fill more easily the Roche lobe, as their radii are larger, there is an additional ''evolutionary'' reason to favor a higher incidence of LMXBs in metal-rich clusters. From the order of magnitude computations made, the new effect by itself could explain the observed ratio of 4 between the frequencies of X-ray clusters in the metal-rich and metal-poor groups we observationally determined. However, there is no reason to exclude that both mechanisms can be at work.