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

TL;DR

This study measures sulfur abundances in three Galactic clusters, revealing low sulfur in Ruprecht 106 suggesting an extragalactic origin, and providing new data for Trumpler 5 and Trumpler 20 to understand sulfur's role in Galactic chemical evolution.

Contribution

First sulfur abundance measurements in Ruprecht 106 and expanded data for Trumpler 5, enhancing understanding of sulfur behavior in different Galactic environments.

Findings

Ruprecht 106 has low sulfur and alpha-element content, indicating extragalactic origin.

Trumpler 5 follows Galactic disk sulfur trends.

Trumpler 20's sulfur content aligns with previous literature.

Abstract

Context. Sulfur (S) is one of the lesser-studied $\alpha$-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 [$\alpha$/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 $\alpha$-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.