The GALAH Survey: Verifying abundance trends in the open cluster M67 using non-LTE spectroscopy

TL;DR

This study uses non-LTE spectroscopy to analyze chemical abundances in 66 stars of the open cluster M67, revealing that non-LTE methods reduce abundance trend discrepancies across different stellar evolutionary stages.

Contribution

First large-scale non-LTE abundance analysis of M67 stars, demonstrating improved consistency in surface abundance trends compared to LTE assumptions.

Findings

Non-LTE analysis reduces star-to-star abundance scatter.

Most abundance trends are flattened with non-LTE, except for Al and Si.

Surface abundance differences correlate with stellar evolutionary stages.

Abstract

Open cluster members are coeval and share the same initial bulk chemical compositions. Consequently, differences in surface abundances between members of a cluster that are at different evolutionary stages can be used to study the effects of mixing and internal chemical processing. We carry out an abundance analysis of seven elements (Li, O, Na, Mg, Al, Si, Fe) in 66 stars belonging to the open cluster M67, based on high resolution GALAH spectra, 1D MARCS model atmospheres, and, for the first time for a large sample of M67 stars, non-local thermodynamic equilibrium (non-LTE) radiative transfer. From the non-LTE analysis, we find a typical star-to-star scatter in the abundance ratios of around 0.05 dex; this scatter is slightly but systematically larger when LTE is assumed instead. We find trends in the abundance ratios with effective temperature, indicating systematic differences in the surface abundances between turn-off and giant stars; these trends are more pronounced when LTE is assumed. However, in the non-LTE analysis, most of the element trends have been flattened. Two species are exceptions to this behaviour, namely Al and Si, which both clearly display remaining trends in the non-LTE analysis. We comment on the possible origin of these trends, by comparing them with recent stellar models that include atomic diffusion.