On the Galactic Chemical Evolution of Sulphur

Sulphur is one of the more interesting chemical elements from a scientific perspective. As a volatile element, sulphur could potentially be used as a tracer of galactic properties, such as star-formation rate and initial mass function, in both the Milky Way and beyond. But first its origin and production has to be understood. Knowledge of Galactic chemical evolution of sulphur is therefore vital. Previous results from spectroscopic analyses of metal-poor stars in our Galaxy have lead to incompatible conclusions: 1) the chemical evolution of sulphur qualitatively follows that of other alpha elements (the standard scenario); 2) large amounts of sulphur were somehow produced in the early Universe, and since then the sulphur production rate has dropped substantially; 3) a combination of the two, indicating a heterogeneous chemical evolution of the Milky Way. In this project we employ the technique of spectrum synthesis to derive sulphur abundances in the atmospheres of a sample of 39 giant and dwarf stars from low (Fe/H ∼ −2.5) to solar metallicities, based on calculated one-dimensional, local thermodynamic equilibriumMARCS models. This study aims to improve on previous studies by: 1) using a homogeneously determined set of stellar parameters to reduce the random errors in the derived abundances; 2) using the forbidden sulphur line at 1082 nm which has only recently become available for analysis and is more robust against some of the assumptions made in the models. We find that 22 of our 39 stars have Fe/H < −1. The sulphur abundances of these stars show general agreement with the standard scenario and with the predictions of contemporary models of Galactic chemical evolution. The continued rise in sulphur abundances towards lower metallicity, as found in some studies, is not confirmed, however, more observations of stars with metallicities between −1.5 and −1.0 are needed to reach more definite conclusions. We conclude that relatively moderate errors in stellar parameters can lead to large errors in sulphur abundances, underscoring the need for accurate stellar parameters. We further find that the forbidden sulphur line can be detected in giant stars with Fe/H > ∼ −2.3 and is found to give lower abundance than other, more model-dependent diagnostics. However, when these model-dependencies are corrected for, other diagnostics give systematically lower abundances than the forbidden sulphur line, suggesting that either these corrections are overestimated or other corrections, likely due to our assumption of one dimension, can not be neglected.