Tests of model predictions for the response of stellar spectra and absorption line indices to element abundance variations

TL;DR

This study tests the accuracy of theoretical response functions used to interpret stellar spectra and absorption lines in galaxy studies, revealing strengths and limitations in current models across different stellar types.

Contribution

The paper evaluates the predictive power of response functions for stellar spectra and line indices using empirical data, highlighting areas needing improvement.

Findings

Iron sensitive line indices are well predicted by response functions.

Balmer line indices are not accurately predicted.

Large scatter observed in other spectral indices.

Abstract

To analyse stellar populations in galaxies a widely used method is to apply theoretically derived responses of stellar spectra and line indices to element abundance variations, hereafter referred to as response functions. These are applied in a differential way, to base models, in order to generate spectra or indices with different abundance patterns. In this paper sets of such response functions for three different stellar evolutionary stages are tested with new empirical [Mg/Fe] abundance data for the MILES stellar spectral library. Recent theoretical models and observations are used to investigate the effects of [Fe/H], [Mg/H] and overall [Z/H] on spectra, via ratios of spectra for similar stars. Global effects of changes in abundance patterns are investigated empirically through direct comparisons of similar stars from the MILES library, highlighting the impact of abundance effects in the blue part of the spectrum, particularly for lower temperature stars. It is found that the relative behaviour of iron sensitive line indices are generally well predicted by response functions, whereas Balmer line indices are not. Other indices tend to show large scatter about the predicted mean relations. Implications for element abundance and age studies in stellar populations are discussed and ways forward are suggested to improve the match with behaviours of spectra and line strength indices observed in real stars.