The forbidden 1082nm line of sulphur: the photospheric abundance of sulphur in the Sun and 3D effects

Context. Sulphur is an element which is formed in the alpha-process and is easily measured in the gaseous phase in external galaxies. Since it does not form dust, it is the preferred indicator for alpha-elements, rather than Si or Mg, for which dust corrections are necessary. The measurement of the sulphur abundance in stars is not an easy task, relying mainly on high excitation lines with non-negligible deviations from LTE. The 1082 nm sulphur forbidden transition is less sensitive to departures from LTE and is less dependent on temperature uncertainties than other sulphur lines usually employed as abundance indicators. Therefore it should provide a more robust abundance diagnostics. Aims. To derive the solar photospheric abundance of sulphur from the 1082 nm [ SI] line and to investigate 3D effects present in G- and F-type atmospheres at solar and lower metallicity. Methods. High-resolution, high signal-to-noise solar intensity and flux spectra were used to measure the sulphur abundance from the [ SI] 1082 nm line. CO(5)BOLD hydrodynamical model atmospheres were applied to predict 3D abundance corrections for the [ SI] line. Results. The solar sulphur abundance is derived to be 7.15 +/-(0.01)(stat) +/-(0.05)(sys), where the statistical uncertainty represents the scatter in the determination using four different solar spectra and the systematic uncertainty is due to the modelling of the blending lines. Sulphur abundances obtained from this line are insensitive to the micro-turbulence. 3D abundance corrections, found from strictly differential comparisons between 1D and 3D models, are negligible in the Sun, but become sizable for more metal-poor dwarfs.