Sulphur abundances in halo giants from the [S I] line at 1082 nm and the [S I] triplet around 1045 nm

Context. It is still debated whether or not the Galactic chemical evolution of sulphur in the halo follows the flat trend with [Fe/H] that is ascribed to the result of explosive nucleosynthesis in type II SNe. It has been suggested that the disagreement between different investigations of sulphur abundances in halo stars might be owing to problems with the diagnostics used, that a new production source of sulphur might be needed in the early Universe, like hypernovae, or that the deposition of supernova ejecta into the interstellar medium is time-delayed. Aims. The aim of this study is to try to clarify this situation by measuring the sulphur abundance in a sample of halo giants using two diagnostics: the S I triplet around 1045 nm and the [S I] line at 1082 nm. The latter of the two is not believed to be sensitive to non-LTE effects. We can thereby minimize the uncertainties in the diagnostic used and estimate the usefulness of the triplet for the sulphur determination in halo K giants. We will also be able to compare our sulphur abundance differences from the two diagnostics with the expected non-LTE effects in the 1045 nm triplet previously calculated by others. Methods. High-resolution near-infrared spectra of ten K giants were recorded using the spectrometer CRIRES mounted at VLT. Two standard settings were used, one covering the S I triplet and one covering the [S I] line. The sulphur abundances were individually determined with equivalent widths and synthetic spectra for the two diagnostics using tailored 1D model atmospheres and relying on non-LTE corrections from the litterature. Effects of convective inhomogeneities in the stellar atmospheres are investigated. Results. The sulphur abundances derived from both the [S I] line and the non-LTE corrected 1045 nm triplet favor a flat trend for the evolution of sulphur. In contrast to some previous studies, we saw no high values of [S/Fe] in our sample. Conclusions. We corroborate the flat trend in the [S/Fe] vs. [Fe/H] plot for halo stars found in some previous studies but do not find a scatter or a rise in [S/Fe] as obtained in other works. We find the sulphur abundances deduced from the non-LTE corrected triplet to be somewhat lower than the abundances from the [S I] line, possibly indicating too large non-LTE corrections. Considering 3D modeling, however, they might instead be too small. Moreover, we show that the [S I] line can be used as a sulphur diagnostic down to [Fe/H] similar to - 2.3 in giants.