The Hamburg/ESO R-process Enhanced Star survey (HERES) VI. The Galactic Chemical Evolution of Silicon

TL;DR

This study measures silicon abundances in 253 metal-poor stars using NLTE calculations, revealing a trend of increasing [Si/Fe] with decreasing metallicity and suggesting insights into early Galactic chemical evolution.

Contribution

It provides a large, NLTE-based silicon abundance dataset for metal-poor stars, clarifying the [Si/Fe] trend and its implications for Galactic chemical evolution models.

Findings

[Si/Fe] increases as metallicity decreases.

Small intrinsic scatter suggests well-mixed supernova yields.

Two stars are identified as highly silicon-enhanced and carbon-rich.

Abstract

We determined the silicon abundances of 253 metal-poor stars in the metallicity range $-4<\mathrm{[Fe/H]} <-1.5$, based on non-local thermodynamic equilibrium (NLTE) line formation calculations of neutral silicon and high-resolution spectra obtained with VLT-UT2/UVES. The $T_{\mathrm{eff}}$ 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 $\mathrm{[Si/Fe]}\sim +1.0$: 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.