What does the "f" mean in O-star classifications?

What does the "f" mean in O-star classifications?

Nolan Walborn (1971) introduced a systematic definition to the f-parameter used in the classification of the spectra of O-type stars earlier than O9. These stars have the highest surface effective temperature of all the stars that lie on the main sequence of the Hertzsprung-Russell diagram. He defined the f-parameter in terms of the appearance of various lines in the spectrum of an O-star, as follows:

Of⁺ Si IV 4089, 4116, N III 4634 - 4640 - 4642 and He II 4686 lines all appear in emission.

Of N III 4634 - 4640 - 4642 and He II 4686 lines appear in emission.

O(f) N III 4634 - 4640 - 4642 lines appear in emission; He II 4686 line appears as weak absorption or is "neutralised".

O((f)) N III 4634 - 4640 - 4642 lines appear as weak emission; He II 4686 line appears as strong absorption.

Many O-type stars emit a spectrum that varies with time, so it can be quite interesting to compare Walborn's classification of a particular star with what might be inferred from a spectrum recorded more recently.

Reference

N.R. Walborn (1971), Some spectroscopic characteristics of the OB stars: an investigation of the space distribution of certain OB stars and the reference frame of the classification, Astrophys. J. Suppl. Series, 23: 257-282.

Stephen Boyle, University of London Observatory, University College London (e-mail: sjb at ulo.ucl.ac.uk)
Date of first version of this article: circa November 2005.
Date of latest modification of this article: 12th August 2009.
Copyright © 2009 Stephen Boyle.

Of⁺	Si IV 4089, 4116, N III 4634 - 4640 - 4642 and He II 4686 lines all appear in emission.
Of	N III 4634 - 4640 - 4642 and He II 4686 lines appear in emission.
O(f)	N III 4634 - 4640 - 4642 lines appear in emission; He II 4686 line appears as weak absorption or is "neutralised".
O((f))	N III 4634 - 4640 - 4642 lines appear as weak emission; He II 4686 line appears as strong absorption.