Evolution of Binaries in Dense Stellar Systems

Abstract

Abstract. In contrast to the field, the binaries in dense stellar systems are frequently not primordial, and could be either dynamically formed or significantly altered from their primordial states. Destruction and formation of binaries occur in parallel all the time. The destruction, which constantly removes soft binaries from a binary pool, works as an energy sink and could be a reason for cluster entering the binary-burning phase. The true binary fraction is greater than observed, as a result, the observable binary fraction evolves differently from the predictions. Combined measurements of binary fractions in globular clusters suggest that most of the clusters are still core-contracting. The formation, on other hand, affects most the more evolutionary advanced stars, which significantly enhances the population of X-ray sources in globular clusters. The formation of binaries with a compact objects proceeds mainly through physical collisions, binary-binary and single-binary encounters; however, it is the dynamical formation of triples and multiple encounters that principally determine whether the formed binary will become an X-ray source.Keywords: binary stars -globular clusters -X-ray binaries -multiple stars -neutron stars -black holes PACS: 97.60.Gb,97.60.Lf,97.60.Jd, BINARIES EVOLUTION: DESTRUCTIONFor low density environments, like solar neighborhood and for solar-type stars, the binary fraction is known to be ∼ 50% (e.g., [1] that of open clusters -e.g., in a core-collapsed NGC 6752, the upper limit for the binary fraction can be as high as 38% [3] (although see also [4]); in a young and sparse globular cluster in Ter 7, the binary fraction is estimated to be 51% [5]. For globular clusters, the measured binary fraction was found to anti-correlate with the total cluster luminosity [6]; the correlation with the cluster collisional parameter was found to be only marginal. All together, measurements of binary fractions in open and globular clusters suggest that binaries in clusters deplete with time and it happens more efficiently in more massive clusters.There is no theoretical study that would suggest that the initial binary fraction should be significantly different at the moment of star formation between proto-open and proto-globular clusters 1 . The consistency between the initially large binary fraction and currently measured low binary fraction can be explained by the efficient binary destruction that continues throughout the entire cluster evolution.Indeed, let us consider a population of primordial binaries with the initial periods P = 0.1 − 10 7 and flat mass ratio distribution, evolved for 10 Gyr using StarTrack [8]. By this age, significant fraction of initial binaries is destroyed through evolution, mainly due to mergers, or, in case of massive stars, in supernovae (see Fig. 1). If we define the hardness of a binary η as the ratio of the binary binding energy to the kinetic energy that a 0.5M ⊙ object moving with v = √ 3 × 10 km/s has (this energy is about an average kinetic energy of an object i...

Evaluating the merger rate of binary black holes from direct captures and third-body soft interactions using the Milky Way globular clusters

“…Assuming that the GC properties depend only on the radial distance from the center, and ignoring mass distri-2 Also known in the literature as "primordial binaries", see e.g. [68] 3 There may be soft binaries that survive and contribute to the 3rd-body channel. The disruption of hard binaries due to some very energetic interaction giving off two isolated BHs is a rare event, [67,70] .…”