Table 2
Classification scheme for spectral types O2-O4 using the emission line ratio N ivλ4058/ N iiiλ4640 and the strength of He iλ4471, as defined by Walborn et al. (2002).
| Spectral type | Criteria |
|
|
|
| Supergiants | |
| O2 If* | N iv ≫ N iii, no or very weak He i |
| O3 If* | N iv > N iii, very weak or no He i |
| O3.5 If* | N iv ~ N iii, very weak He i |
| O4 If+ | N iv < N iii, weak He i |
|
|
|
| Giants | |
| O2 III(f*) | N iv ≫ N iii, no or very weak He i |
| O3 III(f*) | N iv > N iii, very weak or no He i |
| O3.5 III(f*) | N iv ~ N iii, very weak He i |
| O4 III(f+) | N iv < N iii, weak He i |
|
|
|
| Dwarfs | |
| O2 V((f*)) | N iv ≫ N iii, no He i |
| O3 V((f*)) | N iv > or ~ N iii, very weak He i |
| O3.5 V((f+)) | N iv < N iii, very weak He i |
| O4 V((f+)) | no N iv, weak He i |
Notes. Luminosity classes defined as follows. Supergiants (I): He iiλ4686 in emission; giants (III): He iiλ4686 in weak absorption/ P-Cygni profile; dwarfs (V): He iiλ4686 in strong absorption. Note that the f+ designation recently became obsolete since the Si iv emission at λλ4089 − 4116 has been established as a common feature in normal O-type spectra (Sota et al. 2011).
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