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

Abstract

<i>Aims. <i/>The statistical equilibrium of neutral and ionized silicon in the atmospheres of metal-poor stars is discussed. Non-local thermodynamic equilibrium effects (NLTE) are investigated and the silicon abundances in metal-poor stars determined.<i>Methods. <i/>We have used high resolution, high signal to noise ratio spectra from the UVES spectragraph at the ESO VLT telescope. Line formation calculations of Si i and Si ii in the atmospheres of metal-poor stars are presented for atomic models of silicon including 174 terms and 1132 line transitions. Recent improved calculations of Si i and Si ii photoionization cross-sections are taken into account, and the influence of the free-free quasi-molecular absorption in the Ly<i>α<i/> wing is investigated by comparing theoretical and observed fluxes of metal-poor stars. All abundance results are derived from LTE and NLTE statistical equilibrium calculations and spectrum synthesis methods.<i>Results. <i/>It is found that the extreme ultraviolet radiation is very important for metal-poor stars, especially for the high temperature, very metal-poor stars. The radiative bound-free cross-sections also play a very important role for these stars.<i>Conclusions. <i/>NLTE effects for Si are found to be important for metal-poor stars, in particular for warm metal-poor stars. It is found that these effects depend on the temperature. For warm metal-poor stars, the NLTE abundance correction reaches ~0.2 dex relative to standard LTE calculations. Our results indicate that Si is overabundant for metal-poor stars.