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.