A MUSE study of the inner bulge globular cluster Terzan 9: a fossil record in the Galaxy

H Ernandes,E Bica,E Cantelli,S Ortolani,B Barbuy,S Kamann,L Rossi,B Dias

doi:10.1051/0004-6361/201936431

Abstract

Context. Moderately metal-poor inner bulge globular clusters are relics of a generation of long-lived stars that formed in the early Galaxy. Terzan 9, projected at 4°.12 from the Galactic center, is among the most central globular clusters in the Milky Way, showing an orbit which remains confined to the inner 1 kpc. Aims. Our aim is the derivation of the cluster’s metallicity, together with an accurate measurement of the mean radial velocity. In the literature, metallicities in the range between −2.0 < [Fe/H] < −1.0 have been estimated for Terzan 9 based on color-magnitude diagrams and CaII triplet (CaT) lines. Methods. Given its compactness, Terzan 9 was observed using the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope. The extraction of spectra from several hundreds of individual stars allowed us to derive their radial velocities, metallicities, and [Mg/Fe]. The spectra obtained with MUSE were analysed through full spectrum fitting using the ETOILE code. Results. We obtained a mean metallicity of [Fe/H] ≈ −1.10 ±0.15, a heliocentric radial velocity of vhr = 58.1 ± 1.1 km s−1, and a magnesium-to-iron [Mg/Fe] = 0.27 ± 0.03. The metallicity-derived character of Terzan 9 sets it among the family of the moderately metal-poor Blue Horizontal Branch clusters HP 1, NGC 6558, and NGC 6522.

Highlights

Globular clusters in the central parts of the Galaxy are among the oldest extant stellar populations in the Milky Way (e.g. Barbuy et al 2018a; Kunder et al 2018)
The clusters identified with a moderate metallicity and a blue horizontal branch (BHB) are very old as deduced from propermotion cleaned color–magnitude diagrams (CMDs) for example for NGC 6522 and HP 1 (Kerber et al 2018, 2019)
High-resolution spectroscopic analyses based on CaII triplet (CaT) lines from the literature are available: Armandroff & Zinn (1988) obtained [Fe/H] = −0.99, and from 6 stars, Vásquez et al (2018) report Fe/H] = −1.08±0.14, −1.21±0.15, −1.16±0.21, on the scales of Dias et al (2016), Saviane et al (2012), and their own

Summary

Introduction

Globular clusters in the central parts of the Galaxy are among the oldest extant stellar populations in the Milky Way (e.g. Barbuy et al 2018a; Kunder et al 2018). Terzan 9 is a very compact cluster located at 4◦.12 and 0.7 kpc (Bica et al 2006) from the Galactic center, which is, in the inner bulge volume, and it is among the globular clusters closest to the Galactic center. Terzan 9 appears to show a blue horizontal branch (BHB) in the ground-based color–magnitude diagrams (CMDs) by Ortolani et al (1999). The clusters identified with a moderate metallicity and a BHB are very old as deduced from propermotion cleaned color–magnitude diagrams (CMDs) for example for NGC 6522 and HP 1 (Kerber et al 2018, 2019). A propermotion cleaned CMD for Terzan 9 is presented in Rossi et al (2015), with the cluster proper motions derived. Orbit calculations by Pérez-Villegas et al (2018) reveal that Terzan 9 remains

Objectives

Findings

Discussion

Conclusion