Open clusters in the outer disc studied with GTC/MEGARA

Auner 1 and Berkeley 102

¹ Institut d’Estudis Espacials de Catalunya (IEEC), Gran Capità 2–4, 08034 Barcelona, Spain
² Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB), Martí i Franquès, 1, 08028 Barcelona, Spain
³ Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB), Martí i Franquès, 1, 08028 Barcelona, Spain
⁴ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
⁵ Observatoire de Paris (Meudon) GEPI Batiment 11, Hipparque 5 Place Jules Janssen, 92195 Meudon, France
⁶ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, Plaza Ciencias, 1, 28040 Madrid, Spain
⁷ IPARCOS (Instituto de Física de Partículas y del Cosmos), Facultad de Ciencias Físicas, Ciudad Universitaria, Plaza de las Ciencias, 1, Madrid 28040, Spain

^★ Corresponding author: juan636@fqa.ub.edu

Received: 13 December 2024
Accepted: 10 June 2025

Abstract

Context. Open clusters provide valuable information on stellar nucleosynthesis and the chemical evolution of the Galactic disc because their age and distances can be measured more precisely than for field stars.

Aims. We study the outermost parts of the Milky Way disc using open clusters as tracers. We focus on two clusters at galactocentric distances of about 14 kpc that have never been spectroscopically observed before and are located in largely unexplored regions of the Galaxy.

Methods. We used medium-resolution spectra (R > 18 700) obtained with the MEGARA integral-field unit (IFU) spectrograph at the 10.4 m Gran Telescopio Canarias (GTC) to study red giant star members of the clusters Auner 1 and Berkeley 102. We determined the radial velocities and atmospheric parameters for the member stars, and we updated the ages and distances for these two clusters. Finally, we measured the abundances of six chemical elements, that is, Fe, Ca, Co, Ni, Ba, and Eu.

Results. The two clusters are both old, 3.2 ± 0.7 Ga, are distant, d ~ 8 kpc, and are moderately affected by interstellar extinction, A_V ~ 1.3 mag, because they are located below the Galactic mid-plane, Z_Gal ~ −0.7 kpc. The metallicities of Auner 1, [Fe/H]= −0.30 ± 0.09, and Berkeley 102, [Fe/H]= −0.35 ± 0.06, are compatible with the values of other open clusters that are situated at similar galactocentric radii. This suggests that the azimuthal metallicity varies little. The relative abundance ratios, [X/Fe], also behave as expected, perhaps with the exception of [Ca/Fe], which appears to be slightly enhanced in both clusters, and [Eu/Fe], which is enhanced in Berkeley 102, [Eu/Fe] = 0.64 ± 0.05.

Conclusions. Our results demonstrate that competitive Galactic archaeology is possible with GTC/MEGARA observations in IFU mode. The two studied objects open a new window into the chemical evolution of the outer Galactic disc. More observations of distant (in galactocentric distance and azimuth) open clusters with medium- to high-resolution instruments on 8-10 m-class telescopes are needed to firmly establish the abundance trends of the outermost parts of the Galactic disc.