The Hamburg/ESO R-process Enhanced Star survey (HERES)

VI. The Galactic chemical evolution of silicon^⋆,⋆⋆

¹ Key Laboratory of Optical Astronomy, National Astronomical Observatories, CAS, 20A Datun Road, Chaoyang District, 100012 Beijing, PR China
e-mail: zhanglan@bao.ac.cn
² Graduate University of the Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan District, 100049 Beijing, PR China
³ Sydney Institute for Astronomy (SIfA), School of Physics, The University of Sydney, NSW 2006, Australia
⁴ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
⁵ Division of Astronomy and Space Physics, Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden

Received: 17 June 2010
Accepted: 20 January 2011

Abstract

Aims. To obtain detailed silicon abundances of metal-poor stars, we aim to explore the correlation between the abundance ratios and the stellar parameters and the chemical enrichment of the interstellar medium (ISM).

Methods. 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.

Results. The T_eff dependence of [Si/Fe] noticed in previous investigations is diminished in our abundance analysis owing 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. Intrinsic scatter of [Si/Fe] in our sample is small. 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 may have been pre-enriched by sub-luminous supernovae.

Conclusions. 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.