Photo-chemo-dynamical analysis and the origin of the bulge globular cluster Palomar 6^⋆

¹ Universidade de São Paulo, IAG, Rua do Matão 1226, Cidade Universitária, São Paulo 05508-900, Brazil
e-mail: stefano.souza@usp.br
² Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, Potsdam 14482, Germany
³ Instituto de Astronomía, Universidad Nacional Autónoma de México, A. P. 106, 22800 Ensenada, B. C., México
⁴ Università di Padova, Dipartimento di Astronomia, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
⁵ INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁶ Centro di Ateneo di Studi e Attività Spaziali “Giuseppe Colombo” – CISAS, Via Venezia 15, 35131 Padova, Italy
⁷ Aix Marseille Univ., CNRS, CNES, LAM, 13013 Marseille, France
⁸ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
⁹ Institut de Ciències del Cosmos, Universitat de Barcelona (IEEC-UB), Carrer Martí i Franquès 1, 08028 Barcelona, Spain
¹⁰ Universidade Federal do Rio Grande do Sul, Departamento de Astronomia, CP 15051, Porto Alegre, 91501-970 Brazil

Received: 11 July 2021
Accepted: 25 August 2021

Abstract

Context. Palomar 6 (Pal6) is a moderately metal-poor globular cluster projected towards the Galactic bulge. A full analysis of the cluster can give hints on the early chemical enrichment of the Galaxy and a plausible origin of the cluster.

Aims. The aim of this study is threefold: a detailed analysis of high-resolution spectroscopic data obtained with the UVES spectrograph at the Very Large Telescope (VLT) at ESO, the derivation of the age and distance of Pal6 from Hubble Space Telescope (HST) photometric data, and an orbital analysis to determine the probable origin of the cluster.

Methods. High-resolution spectra of six red giant stars in the direction of Pal6 were obtained at the 8 m VLT UT2-Kueyen telescope equipped with the UVES spectrograph in FLAMES+UVES configuration. Spectroscopic parameters were derived through excitation and ionisation equilibrium of Fe I and Fe II lines, and the abundances were obtained from spectrum synthesis. From HST photometric data, the age and distance were derived through a statistical isochrone fitting. Finally, a dynamical analysis was carried out for the cluster assuming two different Galactic potentials.

Results. Four stars that are members of Pal 6 were identified in the sample, which gives a mean radial velocity of 174.3  ±  1.6 km s⁻¹ and a mean metallicity of [Fe/H] = −1.10 ± 0.09 for the cluster. We found an enhancement of α-elements (O, Mg, Si, and Ca) of 0.29< [X/Fe] < 0.38 and the iron-peak element Ti of [Ti/Fe] ∼ +0.3. The odd-Z elements (Na and Al) show a mild enhancement of [X/Fe] ∼ +0.25. The abundances of both first- (Y and Zr) and second-peak (Ba and La) heavy elements are relatively high, with +0.4 <   [X/Fe]  <  +0.60 and +0.4 <   [X/Fe]  <  +0.5, respectively. The r-element Eu is also relatively high with [Eu/Fe] ∼ +0.6. One member star presents enhancements in N and Al, with [Al/Fe] > +0.30, this being evidence of a second stellar population, further confirmed with the NaON-Al (anti)correlations. For the first time, we derived the age of Pal 6, which resulted to be 12.4 ± 0.9 Gyr. We also found a low extinction coefficient R_V = 2.6 for the Pal 6 projection, which is compatible with the latest results for the highly extincted bulge populations. The derived extinction law results in a distance of 7.67 ± 0.19 kpc from the Sun with an A_V = 4.21 ± 0.05. The chemical and photometric analyses combined with the orbital-dynamical analyses point out that Pal 6 belongs to the bulge component probably formed in the main-bulge progenitor.

Conculsions. The present analysis indicates that the globular cluster Pal 6 is located in the bulge volume and that it was probably formed in the bulge in the early stages of the Milky Way formation, sharing the chemical properties with the family of intermediate metallicity very old clusters M 62, NGC 6522, NGC 6558, and HP 1.