The Peculiar Radial Distribution of Multiple Populations in the Massive Globular Cluster M80

Abstract

We present a detailed analysis of the radial distribution of light-element multiple populations (LE-MPs) in the massive and dense globular cluster M 80 based on the combination of UV and optical Hubble Space Telescope data. Surprisingly, we find that first generation stars (FG) are significantly more centrally concentrated than extreme second generation ones (SG) out to ∼ 2.5r h from the cluster center. To understand the origin of such a peculiar behavior, we used a set of N-body simulations following the long-term dynamical evolution of LE-MPs. We find that, given the advanced dynamical state of the cluster, the observed difference does not depend on the primordial relative distributions of FG and SG stars. On the contrary, a difference of ∼ 0.05 − 0.10M between the average masses of the two sub-populations is needed to account for the observed radial distributions. We argue that such a mass difference might be the result of the higher He abundance of SG stars (of the order of ∆Y ∼ 0.05 − 0.06) with respect to FG. Interestingly, we find that a similar He variation is necessary to reproduce the horizontal branch morphology of M 80. These results demonstrate that differences in mass among LE-MPs, due to different He content, should be properly taken into account for a correct interpretation of their radial distribution, at least in dynamically evolved systems.

The Not So Simple Stellar System ω Cen. II. Evidence in Support of a Merging Scenario

“…Therefore, these findings make ω Cen one of the few stellar systems currently known where metal-rich stars have a more extended spatial distribution compared to metal-poor stars. A similar behavior has been observed in the cluster M 80, where Dalessandro et al (2018) found that the stellar subpopulation enriched in Sodium (Na) and depleted in Oxygen (O), which they identify as the second generation, has a more extended spatial distribution compared to the cluster main stellar sub-population (the first generation), which is Na-poor and O-rich. The authors claimed that the two stellar sub-populations have a different helium content and this causes a mass difference, resulting in spatial segregation of the stars, with the lower mass second-generation stars having a more extended spatial distribution.…”

The peculiar kinematics of the multiple populations in the globular cluster Messier 80 (NGC 6093)

“…In light of the results of Dalessandro et al (2018b), who used artificial star tests to infer that their star counts are complete at a > 95% level, no incompleteness corrections were applied. The resulting surface density profiles for the four populations are shown in Fig 2. As the photometry of Dalessandro et al (2018a) is limited to the central region of the cluster, we complemented our global number density profile with the Gaia data recently presented by de Boer et al (2019). After accounting for a vertical offset, the two profiles were stitched together.…”

“…In this paper, we study the globular cluster NGC 6093 (M80). Dalessandro et al (2018a) recently found the three detected populations to be unusually distributed, with the primordial population being more centrally concentrated than the intermediate (in terms of N-enrichment) population, which in turn is more centrally concentrated than the extreme population. NGC 6093 is considered to be dynamically old (Ferraro et al 2012), in which case the different concentrations are unlikely to be a relic from the formation of the cluster.…”

“…NGC 6093 is considered to be dynamically old (Ferraro et al 2012), in which case the different concentrations are unlikely to be a relic from the formation of the cluster. Instead, Dalessandro et al (2018a) suggested that helium variations of ∆Y ∼ 0.05 − 0.06 cause the N-enriched stars to be less massive, so that the different radial distributions can be explained by mass segregation. In this work, we study the kinematics of the populations and investigate if they hold further clues on the dynamical evolution of NGC 6093.…”

“…In this work, we study the kinematics of the populations and investigate if they hold further clues on the dynamical evolution of NGC 6093. With this aim, we combine the photometry of Dalessandro et al (2018a) with MUSE (Bacon et al 2010) integral field spectroscopy.…”