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The modern-era spectral classification of O-stars relies on either theWalborn or the Conti-Mathys scheme. Since both of these approaches have beendeveloped using low-quality photographic data, their application tohigh-quality digital data might not be straightforward and be hampered byproblems and complications that have not yet been appreciated. Usinghigh-resolution spectra obtained with the ESO/MPG 2.2\,m telescope in La Sillaand following the premises of the Walborn and Conti classification schemes, wedetermined the spectral types and luminosity classes of 19 Galactic O-typestars and compared them to those attributed by Walborn and Mathys based onlow-quality data. Our analysis reveals that the morphological spectral typesassigned using high-resolution data are systematically later (by up to 1.5subtypes) then those attributed by Walborn. By means of line-profilesimulations, we show that part of this discrepancy is more likely caused by thecombined effect of stellar rotation and high spectral resolution on the depthof helium lines used as spectral type indicators. In addition, we demonstratethat at least for narrow-lined stars the "rotational effect" does not disappearwhen the high-resolution spectra are degraded to the resolution of the Walbornstandards. We also find evidence of a systematic difference between ourhigh-resolution quantitative spectral types and those assigned by Mathys.Rotation and spectral resolution are important third parameters in the spectralclassification of O-type stars. To obtain reliable spectral classes within theWalborn approach, the unknown and the standard spectra must be compared at thesame resolution and \vsini. Owing to resolution effects, the Conti approachmight also need to be updated.

astronomy and astrophysics

Spectroscopic and physical parameters of Galactic O-type stars. I.  Effects of rotation and spectral resolving power in the spectral  classification of dwarfs and giants