Encounters between compact stars and red giants in dense stellar systems

We present the results of some representative N-body simulations of encounters between a red giant star and a compact object. We have performed a preliminary survey of a large number of such encounters, varying all relevant parameters over wide ranges. We give an overview of all the different possible outcomes that can be produced by the encounters. Our combined analytic and numerical treatment is very general, but we pay particular attention to those encounters which lead to the formation of a binary system. In particular, we discuss how the numerical data on individual encounters can be used to calculate the binary formation rate in globular clusters, and we argue that the usual simple estimates based on the tidal approximation can sometimes give incorrect results. We also discuss the calculation of probability distributions for the semimajor axes and eccentricities of the binaries that are formed, as a function of globular cluster parameters. We show that encounters with stars on the subgiant branch might very well lead to the formation of close binaries (a ∼ 1 R_⊙) with large eccentricity, such as those recently reported in 47 Tue and M15. In addition, the binaries resulting from encounters with stars on the red giant branch could be an important source of heating during the late stages of evolution of a globular cluster.