Bayesian Analysis of Two Stellar Populations in Galactic Globular Clusters. Ii. NGC 5024, NGC 5272, and NGC 6352

Abstract

We use Cycle 21 Hubble Space Telescope (HST) observations and HST archival Advanced Camera for Surveys Treasury observations of Galactic Globular Clusters to find and characterize two stellar populations in NGC 5024 (M53), NGC 5272 (M3), and NGC 6352. For these three clusters, both single and double-population analyses are used to determine a best fit isochrone(s). We employ a sophisticated Bayesian analysis technique to simultaneously fit the cluster parameters (age, distance, absorption, and metallicity) that characterize each cluster. For the twopopulation analysis, unique population level helium values are also fit to each distinct population of the cluster and the relative proportions of the populations are determined. We find differences in helium ranging from ∼0.05 to 0.11 for these three clusters. Model grids with solar α-element abundances ([α/Fe]=0.0) and enhanced α-elements ([α/Fe]=0.4) are adopted.

The ACS survey of Galactic globular clusters – XIV. Bayesian single-population analysis of 69 globular clusters

“…BASE-9 allows us to fix one or more parameters in this way, to use these externally-derived parame-ters when specifying prior distributions, or, if the data are rich enough, to fit the cluster's parameters simultaneously. Our results presented here also provide a baseline measurement to compare to multiple population studies using similar Bayesian methods Wagner-Kaiser et al 2016a).…”

“…Although for our purposes helium is a cluster-level parameter, it is treated separately because it is allowed to vary within the cluster in other contexts Wagner-Kaiser et al 2016a). As in van Dyk et al (2009), binaries can be included in the model by treating each star as a binary system and fitting the component masses.…”

Bayesian analysis of two stellar populations in Galactic globular clusters– III. Analysis of 30 clusters

“…Unlike in Wagner-Kaiser et al (2016b, submitted) where a single population and a single helium abundance are assumed, the current work operates with two separate populations that are distinct in helium. Analyzed with two populations, we find the clusters tend to be younger and at marginally greater distances than in the single population analysis of Wagner-Kaiser et al (2016b, submitted). The additional information provided by the three ultraviolet filters necessarily affects the resulting posterior distribution for cluster and population parameters (Hills et al 2015).…”

“…In Table 3, we present the offsets in the errorweighted medians 2 with standard deviations in comparison to Wagner-Kaiser et al (2016b, submitted), Dotter et al (2010), and Harris (2010). While most of the standard deviations are greater than the offsets, comparisons to Dotter et al (2010) and Harris (2010) suggest that including the ultraviolet observations leads to younger ages.…”

“…We use the sensitive ultraviolet photometry from Piotto et al (2015) as well as visual photometry from Sarajedini et al (2007) to investigate the properties of 30 of the Galactic globular clusters. As presented in Stenning et al (2016) and Wagner-Kaiser et al (2016), we employ a sophisticated Bayesian technique to fit theoretical models to the observed data. In the past, isochrones have primarily been fit to observations of star clusters by eye, trying different parameters and nudging isochrones in color-magnitude space until a solution looks good (Jeffery et al 2016, in press).…”