The Hamburg/ESO R-process Enhanced Star survey (HERES) VI. The Galactic chemical evolution of silicon

Aims. To obtain detailed silicon abundances of metal-poor stars, we aim to explore the correlation between the abundance ratios and the stellar parameters and the chemical enrichment of the interstellar medium (ISM). Methods. We determined the silicon abundances of 253 metal-poor stars in the metallicity range -4 < [Fe/H] < -1.5, based on non-local thermodynamic equilibrium (NLTE) line formation calculations of neutral silicon and high-resolution spectra obtained with VLT-UT2/UVES. Results. The T-eff dependence of [Si/Fe] noticed in previous investigations is diminished in our abundance analysis owing to the inclusion of NLTE effects. An increasing slope of [Si/Fe] towards decreasing metallicity is present in our results, in agreement with Galactic chemical evolution models. Intrinsic scatter of [Si/Fe] in our sample is small. We identified two dwarfs with [Si/Fe] similar to + 1.0: HE 0131-3953, and HE 1430-1123. These main-sequence turnoff stars are also carbon-enhanced. They may have been pre-enriched by sub-luminous supernovae. Conclusions. The small intrinsic scatter of [Si/Fe] in our sample may imply that these stars formed in a region where the yields of type II supernovae were mixed into a large volume, or that the formation of these stars was strongly clustered, even if the ISM was enriched by single SNa II in a small mixing volume.