Metallicity effects in the spectral classification of O-type stars. Theoretical consideration

TL;DR

This study investigates how metallicity influences spectral classification of O-type stars using advanced NLTE models, revealing partial explanations for temperature variations and potential biases in star classification at low metallicity.

Contribution

It provides the first detailed theoretical analysis of metallicity effects on Helium line strengths in O-star spectra using the FASTWIND code.

Findings

Metallicity decrease explains only part of the observed temperature increase in O dwarfs.

Using He II 4686 line may bias classification towards more low-luminosity O-stars at low metallicity.

Metallicity effects alone cannot account for all spectral type variations in O stars.

Abstract

Based on an exteded grid of NLTE, line blanketed model atmospheres with stellar winds as calculated by means of FASTWIND, we have investigated the change in the strengths of strategic Helium transitions in the optical as caused by a 0.3 decrease in metallicity with respect to solar abundances. Our calculations predict that only part of the observed increase in Teff, of O-type dwarfs could be explained by metallicity effects on the spectral type indicators, while the rest must be attributed to other reasons (e.g., different stellar structures as a function of metallicity or differences between observed and theoretical wind parameters etc.). In addition, we found that using the He II 4686 line to classify stars in low metallicity environments (Z < 0.3 solar) might artificially increase the number of low luminosity (dwarfs and giants) O-stars, on the expense of the number of O-supergiants.