High-resolution abundance analysis of four red giants in the globular cluster NGC 6558

Barbuy, B.; Muniz, L.; Ortolani, S.; Ernandes, H.; Dias, B.; Saviane, I.; Kerber, Leandro; Bica, E.; Pérez-Villegas, Ángeles; Rossi, Luca; Held, E. V.

doi:10.1051/0004-6361/201833953

Cited by 34 publications

(28 citation statements)

References 102 publications

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“…This scenario is further supported by the fact that some of the oldest globular clusters in the Galaxy (such as NGC 6522, NGC 6558 and HP1) reside in the bulge and have metallicities of [Fe/H] ∼ −1.0 (e.g. Barbuy et al 2009Barbuy et al , 2018bOrtolani et al 2011;Kerber et al 2019). The properties of old globular clusters in the bulge are a nice and independent confirmation of the results presented in this manuscript.…”

Section: Discussionsupporting

confidence: 83%

The age of the Milky Way inner stellar spheroid from RR Lyrae population synthesis

Savino

Koch

Prudil

et al. 2020

A&A

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The central kiloparsecs of the Milky Way are known to host an old, spheroidal stellar population, whose spatial and kinematical properties set it apart from the boxy-peanut structure that constitutes most of the central stellar mass. The nature of this spheroidal population, whether it is a small classical bulge, the innermost stellar halo, or a population of disk stars with large initial velocity dispersion, remains unclear. This structure is also a promising candidate to play host to some of the oldest stars in the Galaxy. Here we address the topic of the inner stellar spheroid age, using spectroscopic and photometric metallicities for a sample of 935 RR Lyrae stars that are constituents of this component. By means of stellar population synthesis, we derive an age-metallicity relation for RR Lyrae populations. We infer, for the RR Lyrae stars in the bulge spheroid, an extremely ancient age of 13.41 ± 0.54 Gyr and conclude they were among the first stars to form in what is now the Milky Way galaxy. Our age estimate for the central spheroid shows a remarkable agreement with the age profile that has been inferred for the Milky Way stellar halo, suggesting a connection between the two structures. However, we find mild evidence for a transition in the halo properties at rGC ∼ 5 kpc. We also investigate formation scenarios for metal-rich RR Lyrae stars, such as binarity and helium variations, and consider whether they can provide alternative explanations for the properties of our sample. We conclude that within our framework, the only viable alternative is to have younger, slightly helium-rich, RR Lyrae stars. This is a hypothesis that would open intriguing questions for the formation of the inner stellar spheroid.

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Section: Discussionsupporting

confidence: 83%

The age of the Milky Way inner stellar spheroid from RR Lyrae population synthesis

Savino

Koch

Prudil

et al. 2020

A&A

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“…The source ESO456-SC38 is an addition to the list of moderately metal-poor ([Fe/H]∼ −1.0) clusters on the blue horizontal branch, and is projected very close to the Galactic center. It appears to be similar to NGC 6522 (Barbuy et al 2014, Kerber et al 2018, HP 1 , Kerber et al 2019, and NGC 6558 (Barbuy et al 2018b). The Galactic bulge currently appears to be dominated by a bar, but in its early beginnings, a classical bulge was probably present, as proven by the concentration of these very old GCs.…”

Section: Discussionsupporting

confidence: 71%

Another relic bulge globular cluster: ESO 456-SC38 (Djorgovski 2)

Ortolani

Held

Nardiello

et al. 2019

A&A

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Context. The object ESO456-SC38 (Djorgovski 2) is one of the globular clusters that is closest to the Galactic center. It is on the blue horizontal branch and has a moderate metallicity of [Fe/H]∼-1.0. It is thus similar to the very old inner bulge globular clusters NGC 6522, NGC 6558, and HP 1, and therefore appears to be part of the primeval formation stages of the Milky Way. Aims. The aim of this work is to determine an accurate distance and metallicity for ESO456-SC38, as well as orbital parameters, in order to check similarities with other clusters in the inner bulge that have previously been well studied in terms of spectroscopy and photometry. This is a considerably fainter cluster that is contaminated by a rich stellar field; it is also quite absorbed by the dusty foreground. Methods. We analyzed ESO456-SC38 based on Hubble Space Telescope photometry, with the filters F606W from ACS, F110W and F160W from WFC3, and photometry in V and I from FORS2 at the VLT. We combined this with identified stars that are covered by Gaia Data Release 2.Results. The isochrone fitting was carried out with the statistical Markov chain Monte Carlo method. We derive an accurate distance of d = 8.75 ± 0.12 kpc and a reddening of E(B-V)= 0.81 +0.02 −0.02 . The best-fitting BaSTI isochrones correspond to an age of 12.70 +0.72 −0.69 Gyr and a metallicity of [Fe/H]= −1.11 +0.03 −0.03 . Conclusions. ESO 456-SC38 adds to the list of moderately metal-poor globular clusters located in the inner bulge. It is on the blue horizontal branch and is very old. The cluster is confined to the bulge and bar region, but it does not support the Galactic bar structure. The old stellar population represented by clusters like this has to be taken into account in models of Galactic bulge formation. Studying them also provides indications on the formation times of the globular clusters themselves.

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“…The result indicated that the most reliable region is 6000-6800 Å, which is, in fact, the region commonly used to derive stellar parameters from high-resolution spectra (e.g. Barbuy et al 2018c). This is explained by the following facts: it is widely known that when bluer than 6000 Å, the continuum is progressively affected by molecular lines as well as a large number of faint lines.…”

Section: Stellar Parametersmentioning

confidence: 94%

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

Ernandes

Dias

Barbuy

et al. 2019

A&A

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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 v h r = 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.

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High-resolution abundance analysis of four red giants in the globular cluster NGC 6558

Cited by 34 publications

References 102 publications

The age of the Milky Way inner stellar spheroid from RR Lyrae population synthesis

The age of the Milky Way inner stellar spheroid from RR Lyrae population synthesis

Another relic bulge globular cluster: ESO 456-SC38 (Djorgovski 2)

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

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