The ACS survey of Galactic globular clusters

Abstract

Context. The fraction of binary stars is an important ingredient to interpret globular cluster dynamical evolution and their stellar population. Aims. We investigate the properties of main-sequence binaries measured in a uniform photometric sample of 59 Galactic globular clusters that were observed by HST WFC/ACS as a part of the Globular Cluster Treasury project. Methods. We measured the fraction of binaries and the distribution of mass-ratio as a function of radial location within the cluster, from the central core to beyond the half-mass radius. We studied the radial distribution of binary stars, and the distribution of stellar mass ratios. We investigated monovariate relations between the fraction of binaries and the main parameters of their host clusters. Results. We found that in nearly all the clusters, the total fraction of binaries is significantly smaller than the fraction of binaries in the field, with a few exceptions only. Binary stars are significantly more centrally concentrated than single MS stars in most of the clusters studied in this paper. The distribution of the mass ratio is generally flat (for mass-ratio parameter q > 0.5). We found a significant anti-correlation between the binary fraction in a cluster and its absolute luminosity (mass). Some, less significant correlation with the collisional parameter, the central stellar density, and the central velocity dispersion are present. There is no statistically significant relation between the binary fraction and other cluster parameters. We confirm the correlation between the binary fraction and the fraction of blue stragglers in the cluster.

Testing multimass dynamical models of star clusters with real data: mass segregation in three Galactic globular clusters

“…A cluster-to-cluster comparison between the fraction of binaries estimated here and those listed in Milone et al (2012) indicate no significant differences, although they are all systematically smaller than those estimated by these authors by ∆f b ∼ 1.8 − 5.2%. Consider that the analysis of Figure 5.…”

“…A population of binaries has been also simulated by associating to a fraction of stars a secondary component whose mass has been randomly extracted assuming a flat massratios distribution (Milone et al 2012). The fluxes of the two components have been then summed in both passbands to derive their corresponding magnitudes and color.…”

The M 4 Core Project with HST: I. Overview and first epoch

“…For many years, searches for binaries in clusters revealed only a very small binary fraction, which led theorists to conclude that there were almost no primordial binaries (or none left) and that binaries would only be formed in the high densities reached at the end of core collapse, heralding a succession of core oscillations as the cluster steadily lost stars across the Galactic tidal boundary. Recently, as we have been able to search for binaries well into the core using multi-wavelength photometric techniques, it is clear that they are quite numerous (see Milone et al 2012a, and references therein), and we had previously missed them because they had sunk to the core. The presence of primordial binaries in clusters has a significant effect on our dynamical models.…”

A young cluster with an extended main-sequence turnoff: confirmation of a prediction of the stellar rotation scenario

“…We have estimated the amount of differential extinction in the NGC 1850 field using the technique of Milone et al (2012). As the extinction vector is largely parallel to the main sequence (MS; 18.5 < F 438W < 20.5) we used the lower part of the MS.…”