NLTE Analysis Of Si Abundance In Stars Of The Solar Neighbourhood

Formation and evolution of our Galaxy are very important astronomy physical problem.The chemical element abundance provides information of evolution of the Galaxy directly.Stellar chemical abundances analysis is the basic means of exploring the evolution of stars,the Milky Way, and even the universe. This paper mainly studied the Si abundance for tworeasons: Si elements as a typical elements can be using as a tracer element to study thechemical evolution stars; Si belongs to the most abundant metals. It competes with Mg and Fe,which also have similar ionisation energies. It is therefore important as an electron donor inrelatively cool turnoff stellar atmospheres. The high-resolution spectra analyzed in our workwere obtained with the FOCES fiber optics echelle spectrograph fed by the2.2m telescope ofthe DSAZ at Calar Alto Observatory during August2001and January2003.The previous abundance determinations of Si have been carried out under the LTEassumption. The influence of the NLTE effects is very complex. The influence is different forstars with ifferent metallicities. Our analyses are based on a sample of166stars（include somemetal-rich stars）and aims at exploring their [Si/Fe] abundance ratios vs Teffof metal-poor; thetrend of [Si/Fe] vs [Fe/H] of metal-rich stars,and the separation between thin disk and thickdisk starts by kinematics and [Mg/Fe].The first part is introduction, and the observational technique are described in Section2.In Sect.3we present the atmospheric models and stellar parameters. The NLTE formationand atomic model of silicon is discussed in Sect.4. The results and the analysis are presentedin5. The discussion is shown in Section6, while the conclusions are given in Sect.7.Based on our results we come to the following conclusions:（1）The NLTE effects increase with increasing temperature, which means that theNLTE effects are large for hot stars.（2）The effects of NLTE are different for different ionization stages of Si. The crrectionof NLTE for Si I is too small to be considered, while it is more obvious for Si II which adds to-0.06dex. As a result, the average discrepancy of Si abundance derived from Si I and Si IIions decreased to0.02dex from0.07dex under LTE assumption. （3）For metal-poor stars,[Si/Fe] decreased with [Fe/H] increasing, but for metal-richstars,[Si/Fe] slightly increases with increasing [Fe/H].（4）The abundance ratio [Si/Fe] in the thick disk stars is obviously higher than that inthe thin disk stars, but there is not a exact boundary in Si abundance to distinguish clearly thethin and thick disk stars.（5）Although both Si and Mg are-elemnts and are mainly produced during Type IIsupernovae explosions of massive stars at early stage of the Galactic evolution, they may havedifferent nucleosynthesis machnism at later stage of the Galactic evolution when someproduction of Si maybe contributed by Type Ia supernovae and AGB stars.