Quantitative spectral classification of Galactic O stars

TL;DR

This paper provides a detailed quantitative framework for classifying Galactic O stars based on spectral line measurements, improving the precision and consistency of spectral and luminosity classifications.

Contribution

It introduces new quantitative criteria for spectral and luminosity class determination of O stars, based on high-resolution spectral data and equivalent width ratios.

Findings

Ratios of HeI 4471 to HeII 4542 match published spectral type ratios.

Quantified equivalent width ratios refine spectral typing in O8-O9.7 range.

Criteria effectively distinguish luminosity classes and spectral subclasses.

Abstract

Our goal is to provide a quantification of several spectral classification criteria for O stars. We collected high-spectral resolution spectra of 105 Galactic O-type stars from various archives. We measured equivalent widths of classification lines. We defined average values of classification criteria for given spectral types and luminosity classes. We find that the ratio HeI 4471 to HeII 4542 well matches the published ratios for spectral types. We have quantified equivalent width ratios of helium and silicon lines among O8-O9.7 stars to refine spectral class typing in this spectral range. We present quantitative criteria to separate between luminosity class V, IV-III-II (grouped), and I among O3-O8.5 stars, mainly based on the strength of HeII 4686. We find that these criteria also define very well the f, (f), and ((f)) classes for O3-O7.5 stars. Among O9-O9.7 stars we quantify the ratios of HeII 4686 to HeI 4713 and SiIV 4089 to HeI 4026 for all luminosity classes. The tabulated values of the classification criteria should help classify any new O-type stars. The final step of the classification process should rely on a direct comparison to standard stars of the assigned spectral type and luminosity class.