科研之友
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摘要
Binary-single and binary-binary encounters play a pivotal role in the evolution of star clusters, as they may lead to the disruption or hardening of binaries, a novel prediction of the Hills-Heggie law. Based on our recent Chandra survey of Galactic globular clusters (GCs), we revisit the role of stellar dynamical interactions in GCs, focusing on main-sequence (MS) binary encounters as a potential formation channel of the observed X-ray sources in GCs. We show that the cumulative X-ray luminosity (L-X), a proxy of the total number of X-ray-emitting binaries (primarily cataclysmic variables and coronally active binaries) in a given GC, is highly correlated with the MS binary encounter rate (Gamma(b)), as L-X proportional to Gamma(0.77 +/- 0.11)(b). We further test the Hills-Heggie law against the binary hardness ratio, defined as the relative number of X-ray-emitting hard binaries to MS binaries and approximated by L-X (L-K f(b)), with L-K being the GC K-band luminosity and f(b) the MS binary fraction. We demonstrate that the binary hardness ratio of most GCs is larger than that of the solar neighborbood stars, and exhibits a positive correlation with the cluster specific encounter rate (gamma), as L-X/(L-K f(b)) proportional to gamma(0.65 +/- 0.12). We also find a strong correlation between the binary hardness ratio and cluster velocity dispersion (sigma), with L-X/(L-K f(b)) proportional to sigma(1.71 +/- 0.48) () mu s1.71. 0.48, which is consistent with the Hills-Heggie law. We discuss the role of binary encounters in the context of the Nuclear Star Cluster, arguing that the X-ray-emitting, close binaries detected therein could have been predominantly formed in GCs that later inspiralled to the Galactic center.
