NLTE strontium abundance in a sample of extremely metal poor stars and the Sr/Ba ratio in the early Galaxy

TL;DR

This study refines strontium abundance measurements in extremely metal-poor stars using non-LTE modeling, revealing large scatter and insights into early galaxy neutron-capture processes.

Contribution

It provides the first homogeneous non-LTE abundance analysis of Sr in a large sample of extremely metal-poor stars, improving accuracy over LTE methods.

Findings

Confirmed large scatter of Sr abundance at low metallicities

Found Sr abundance compatible with the main r-process in early stars

Observed even larger scatter at very low metallicities

Abstract

Heavy element abundances in extremely metal-poor stars provide strong constraints on the processes of forming these elements in the first stars. We attempt to determine precise abundances of strontium in a homogeneous sample of extremely metal-poor stars. The abundances of strontium in 54 very or extremely metal-poor stars, was redetermined by abandoning the local thermodynamic equilibrium (LTE) hypothesis, and fitting non-LTE (NLTE) profiles to the observed spectral lines. The corrected Sr abundances and previously obtained NLTE Ba abundances are compared to the predictions of several hypothetical formation processes for the lighter neutron-capture elements. Our NLTE abundances confirm the previously determined huge scatter of the strontium abundance in low metallicity stars. This scatter is also found (and is even larger) at very low metallicities (i. e. early in the chemical evolution). The Sr abundance in the extremely metal-poor (EMP) stars is compatible with the main r-process involved in other processes (or by variations of the r-process), as we briefly discuss.