Sulfur abundances in three Galactic clusters: Ruprecht 106, Trumpler 5, and Trumpler 20^★

¹ Departamento de Ciencias Fisicas, Faculdad de Ciencias Exactas, Universidad Andres Bello, Av. Fernandez Concha 700, Las Condes, Santiago, Chile
e-mail: lucertini.fra@gmail.com
² ESO – European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
³ GEPI, Observatoire de Paris, Université PSL, CNRS, Place Jules Janssen, 92195 Meudon, France
⁴ Dipartimento di Fisica e Astronomía, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁵ INAF – Osservatorio di Astrofísica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁶ Departamento de Astronomia, Casilla 160-C, Universidad de Concepcion, Cooncepción, Chile

Received: 31 July 2022
Accepted: 5 October 2022

Abstract

Context. Sulfur (S) is one of the lesser-studied α-elements. Published investigations of its behavior have so far focused on local stars, and only a few clusters of the Milky Way have been considered to study this topic. We aim to study the S content of the globular cluster Ruprecht 106 – which has never before been studied for this purpose, but is known to present low levels of the [α/Fe] abundance ratio – and the open cluster Trumpler 5. The only star studied so far in Trumpler 5 shows an unexpectedly low abundance of S.

Aims. With this work, we aim to provide the first S abundance in Ruprecht 106 and to investigate the S content of Trumpler 5 with a larger sample of stars. The open cluster Trumpler 20 is considered as a reference object.

Methods. We performed a standard abundance analysis based on 1D model atmospheres in local thermodynamical equilibrium (LTE) and on high-resolution and high-signal-to-noise-ratio UVES-slit and UVES/FLAMES spectra. We also applied corrections for nonLTE. The metallicities of the targets were obtained by studying equivalent widths. Sulfur abundances were derived from multiplets 1, 6, and 8 by spectrosynthesis.

Results. We find that the metallicities of Ruprecht 106 and Trumpler 5 are [Fe/H] = −1.37±0.11 and [Fe/H] = −0.49±0.14, respectively. Ruprecht 106 is less S-rich than the other Galactic clusters at similar metallicity. The low S content of Ruprecht 106, [S/Fe]_NLTE = −0.52±0.13, is consistent with its shortage of α-elements. This supports an extra-galactic origin of this cluster. We obtained a new and more robust S content value of Trumpler 5 of about [S/Fe]_NLTE = 0.05±0.20. According to our results, Trumpler 5 follows the trend of the Galactic disk in the [S/Fe]_LTE versus [Fe/H] diagram. Our results for Trumpler 20, of namely [Fe/H]= 0.06±0.15 and [S/Fe]_NLTE = −0.28±0.21, are in agreement with those in the literature.