NLTE modelling of integrated light spectra

P Eitner,S Larsen,M Bergemann

doi:10.1051/0004-6361/201935416

Abstract

Aims.We study the effects of non-local thermodynamic equilibrium (NLTE) on the abundance analysis of barium, magnesium, and manganese from integrated light spectroscopy, as typically applied to the analysis of extra-galactic star clusters and galaxies. In this paper, our reference object is a synthetic simple stellar population (SSP) representing a mono-metallicα-enhanced globular cluster with the metallicity [Fe/H] = −2.0 and the age of 11 Gyr.Methods.We used the MULTI2.3 program to compute LTE and NLTE equivalent widths of spectral lines of Mg I, Mn I, and Ba II ions, which are commonly used in abundance analyses of extra-galactic stellar populations. We used ATLAS12 model atmospheres for stellar parameters sampled from a model isochrone to represent individual stars in the model SSP. The NLTE and LTE equivalent widths calculated for the individual stars were combined to calculate the SSP NLTE corrections.Results.We find that the NLTE abundance corrections for the integrated light spectra of the metal-poor globular cluster are significant in many cases, and often exceed 0.1 dex. In particular, LTE abundances of Mn are consistently under-estimated by 0.3 dex for all optical lines of Mn I studied in this work. On the other hand, Ba II, and Mg I lines show a strong differential effect: the NLTE abundance corrections for the individual stars and integrated light spectra are close to zero for the low-excitation lines, but they amount to − 0.15 dex for the strong high-excitation lines. Our results emphasise the need to take NLTE effects into account in the analysis of spectra of individual stars and integrated light spectra of stellar populations.

Highlights

The chemical composition of stellar populations provides vital constraints on their formation histories and on the relative contributions of different nucleosynthetic processes to chemical enrichment
The non-local thermodynamic equilibrium (NLTE) abundance corrections for Ba, Mg, and Mn for all model atmospheres in the simple stellar population (SSP) are shown in Fig. 3, where the results are plotted against surface gravity
Our results suggest that this can be explained mostly as a NLTE effect, with the actual [Mn/Fe] ratios most likely being close to solar. our models predict that there is no significant variation in the NLTE abundance correction between the individual Mn I lines, the redder lines at 6016, 6021 Å have somewhat, ∼0.05 dex, larger NLTE correction compared to the bluer line at 4754 Å

Summary

Introduction

The chemical composition of stellar populations provides vital constraints on their formation histories and on the relative contributions of different nucleosynthetic processes to chemical enrichment. Measurements of overall metallicities and metallicity gradients can provide information on the importance of inand outflows, variability of initial mass function and star formation activity, and mergers The ratio of alpha-capture elements (O, Mg, Si, S, Ca) to iron can constrain the relative importance of core-collapse and Type Ia SNe, and the time scales for chemical enrichment The enhancement in the [Ba/Fe] ratio in metal-rich stars in the Large Magellanic Cloud may indicate an important contribution by the s-process via AGB winds (Van der Swaelmen et al 2013; Nidever et al 2019)

Methods

Results

Discussion

Conclusion