Statistical equilibrium of silicon in the atmospheres of nearby metal-poor stars

Aims. We discuss the statistical equilibrium of neutral and ionized silicon in the atmospheres of nearby metal-poor stars. We investigated the effects of non-local thermodynamic equilibrium (NLTE) and determined the silicon abundances. Methods. We used high resolution, high signal-to-noise ratio spectra from the FOCES spectragraph at the DSAZ telescope. Line-formation calculations of Si I and Si II in the atmospheres of nearby metal-poor stars are presented. All abundance results are derived from local thermodynamic equilibrium (LTE) and NLTE statistical equilibrium calculations and spectrum synthesis methods. Results. We find that NLTE effects for Si II optical lines are important for warm stars, and that they depend on effective temperature. The Si abundances of thin and thick disc stars follow distinct trends, as in the case of Mg. We find that [Si/Fe] gradually increases as [Fe/H] decreases in thin disc stars, while it remains around at similar to+0.30 dex for halo and thick disc stars, the halo stars showing larger scatter. Conclusions. The derived dependence between [Si/Fe] and [Fe/H] is inconsistent with the theoretical predictions of published model calculations for the chemical evolution of the Galaxy. The nearly constant [Si/Mg] ratio with some scatter for halo and thick disc stars suggests that the nucleosynthesis of silicon is closely coupled to that of Mg. In addition, our results do not support the suggestion that type Ia supernove produce significant amounts of silicon.