A Fossil Bulge Globular Cluster revealed by VLT Multi-Conjugate Adaptive Optics

TL;DR

This study used advanced adaptive optics at the VLT to analyze the globular cluster HP1 near the Galactic bulge, revealing its properties, age, and orbit, and confirming its status as a fossil bulge cluster.

Contribution

The paper demonstrates the use of VLT multi-conjugate adaptive optics combined with proper motion analysis to study a bulge globular cluster in unprecedented detail.

Findings

Confirmed metallicity of [Fe/H] ~ -1.0

Identified an extended blue horizontal branch

Suggested the cluster is confined within the bulge

Abstract

The globular cluster HP1 is projected on the bulge, very close to the Galactic center. The Multi-Conjugate Adaptive Optics (MCAO) Demonstrator (MAD) at the Very Large Telescope (VLT) allowed to acquire high resolution deep images that, combined with first epoch New Technology Telescope (NTT) data, enabled to derive accurate proper motions. The cluster and bulge field stellar contents were disentangled by means of this process, and produced unprecedented definition in the color-magnitude diagrams for this cluster. The metallicity of [Fe/H] ~ -1.0 from previous spectroscopic analysis is confirmed, which together with an extended blue horizontal branch, imply an age older than the halo average. Orbit reconstruction results suggest that HP1 is spatially confined within the bulge.