The Hamburg/ESO R-process Enhanced Star survey (HERES) VI. The Galactic Chemical Evolution of Silicon

We determined the silicon abundances of 253 metal-poor stars in the metallicity range $-4<[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]<-1.5$, based on non-local thermodynamic equilibrium (NLTE) line formation calculations of neutral silicon and high-resolution spectra obtained with VLT-UT2/UVES. The $T_{\mathrm{e}\mathrm{f}\mathrm{f}}$ dependence of [Si/Fe] noticed in previous investigation is diminished in our abundance analysis due 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. 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. We identified two dwarfs with $[\mathrm{S}\mathrm{i}\mathrm{/}\mathrm{F}\mathrm{e}]\sim +1.0$: HE 0131$-$3953, and HE 1430$-$1123. These main-sequence turnoff stars are also carbon-enhanced. They might have been pre-enriched by sub-luminous supernovae.