Vol. 584
In section 5. Galactic structures

The incidence of binaries in globular cluster stellar populations

by S. Lucatello, A. Sollima, R. Gratton, E. Vesperini, V. D'Orazi, E. Carretta, A. Bragaglia A&A 584, A52

The binary fraction and period distribution of binary systems contains key information about the formation and evolution of stellar systems, especially in dense stellar systems that possibly contain several generations of stars. Obtaining these quantities is an observationally challenging task. Over a period of several years, the above team surveyed almost nine hundred stars located around the half-light radius of ten Galactic globular clusters to search for radial velocity variations. With their data, they were also able to ascertain whether a star compositionally belonged to the first or second generation. They found binary fractions of ~5% and ~1% among first- and second-generation stars, respectively. The authors argue that their findings are robust against observational biases and, in fact, provide support for the differing binary fractions expected to result from the dynamical evolution of stellar systems containing multiple generations of stars, with the second generation of stars forming in a denser environment.