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. The Teff
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
[Si/Fe]∼+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.Comment: 12 pages, 9 figures, 1 electronical table. Accepted by A &
