Abstract
Our goal is to provide a quantification of several spectral classification criteria for O stars. We collect 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 He I 4471 to He II 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 He II 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 He II 4686 to He I 4713 and Si IV 4089 to He I 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 or luminosity class.
Highlights
Massive stars have initial masses higher than about 10 M
A direct comparison to standard stars is mandatory to assess the strength of He ii 4686. This is usually not straightforward since broadening mechanisms, especially rotational broadening, affect the strength of any absorption line. To partially reduce these difficulties, we present a quantitative study of classification criteria for spectral classes and luminosity classes among O stars
We present the quantification of classification criteria for spectral types and luminosity classes
Summary
Massive stars have initial masses higher than about 10 M They are born as O and B stars and evolve into blue, yellow and red supergiants, luminous blue variables or Wolf-Rayet stars. They end their lives as core-collapse supernovae of type II, Ib or Ic, sometimes associated with a long-soft gamma-ray burst. The MK classification system (Morgan et al 1943; Morgan & Keenan 1973) introduced criteria to separate O stars with different ratios of He i to He ii lines. The second dimension of the MK classification – luminosity class – was based on the relative strength if Si iv to He i lines
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