The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters – XXI. Binaries among multiple stellar populations

Abstract

A number of scenarios for the formation of multiple populations in globular clusters (GCs) predict that second generation (2G) stars form in a compact and dense subsystem embedded in a more extended first-generation (1G) system. If these scenarios are accurate, a consequence of the denser 2G formation environment is that 2G binaries should be more significantly affected by stellar interactions and disrupted at a larger rate than 1G binaries. The fractions and properties of binary stars can thus provide a dynamical fingerprint of the formation epoch of multiple-population GCs and their subsequent dynamical evolution. We investigate the connection between binaries and multiple populations in five GCs, NGC 288, NGC 6121 (M 4), NGC 6352, NGC 6362, and NGC 6838 (M 71). To do this, we introduce a new method based on the comparison of Hubble Space Telescope observations of binaries in the F275W, F336W, F438W, F606W and F814W filters with a large number of simulated binaries. In the inner regions probed by our data we do not find large differences between the local 1G and the 2G binary incidences in four of the studied clusters, the only exception being M 4 where the 1G binary incidence is about three times larger than the 2G incidence. The results found are in general agreement with the results of simulations predicting significant differences in the global 1G and 2G incidences and in the local values in the clusters' outer regions but similar incidences in the inner regions. The significant difference found in M 4 is consistent with simulations with a larger fraction of wider binaries. Our analysis also provides the first evidence of mixed (1G-2G) binaries, a population predicted by numerical simulations to form in a cluster's inner regions as a result of stellar encounters during which one component of a binary is replaced by a star of a different population.

Star Clusters Near and Far

“…Additional support for the hot-hydrogen burning hypothesis comes from the fact that some GCs display broadened or split main sequences in the colour magnitude diagram (Anderson 2002;Bedin et al 2004;Piotto et al 2007Piotto et al , 2012Han et al 2009), which has been attributed to He spreads (Norris 2004;D'Antona et al 2005;Charbonnel 2016; Lagioia et al 2019). More recently, the so-called chromosome map was introduced, which is based on specific combinations of HST filters that are sensitive to He and N ; it turns out to be a very powerful photometric tool to detect the presence of multiple stellar populations in Galactic and extra-galactic GCs (Milone et al 2017;Zennaro et al 2019;Saracino et al 2019). In the last few years, evidence for N enhancement has been found in younger clusters ( 2 Gyr) in the Magellanic Clouds (e.g., Hollyhead et al 2017;Martocchia et al 2018a,b).…”

“…As mentioned earlier, the O-Na anti-correlation is so ubiquitous that it has been suggested to be the unique identifying property of a genuine GC (Carretta et al 2010). The ubiquity does not mean that all GCs are similar; quite the contrary, the details of the multiple populations are different in every GC (Milone et al 2017). Some note-worthy trends with GC properties have been identified, that might provide clues to the origin of these multiple populations.…”

“…Both findings are expected if these anti-correlations are the result of hot-H burning and the temperature of the polluter was higher in more massive and metal-poor GCs. In addition, both the He spread inferred from the main sequence broadening and the fraction of stars with anomalous abundances correlate with GC mass (Milone et al 2014(Milone et al , 2017, implying that more polluted material is produced per unit of cluster mass in massive GCs. More details on the observational signatures can be found in Bastian and Lardo (2018) and Gratton et al (2019).…”

Inferring the Helium abundance of extragalactic Globular Clusters using Integrated Spectra

“…In Milone et al (2017), the fraction of first generation stars was shown to anti-correlate with cluster mass and Lucatello et al (2015) showed that the more Helium poor first generation stars have a higher binary fraction compared to second generation stars. However, the results of the recent study by Milone et al (2020) suggest equal binary fractions for the majority of clusters. Also, the internal Helium abundance varies significantly between first and second generation stars for some clusters (Milone et al 2018).…”

Using Binaries in Globular Clusters to Catch Sight of Intermediate-Mass Black Holes

“…Extending our analysis to other detection techniques for binaries will provide a better link between the estimated binary fraction and kinematics effects of binaries. To do so, we first need to consider multiple populations in the simulations (Hong et al 2015) as they play a relevant role in the photometric binary detection (Milone et al 2020). At the same time, it is necessary to check the different biases in the detection methods: radial velocity methods mainly detect binaries with short periods, whereas photometric methods depend on the mass ratio of binaries (Milone et al 2012).…”