On the class of Oe stars

Abstract

We present high-quality spectra of the majority of stars that have been classified as Oe and find that their published spectral types are generally too early, most likely due to infilling of HeI lines. As a matter of fact, all stars classified as Oe actually fall inside the range O9-B0 with the important exception of HD155806 (O7.5III) and perhaps HD39680 (difficult to classify, but likely O8.5V). Observations of a sample of objects with published spectral types in the O9-B0 range previously classified as peculiar or emission-line stars fail to reveal any new Oe star with spectral type earlier than O9.5. Most objects classified as peculiar in ``classical'' literature show signs of binarity in our spectra, but no spectral anomalies. We conclude that there is likely a real decline in the fraction of Be stars for spectral types earlier than B0, not due to observational bias. The few Oe stars with spectral types earlier than O9.5 deserve detailed investigation in order to provide constraints on the physical reasons of the Be phenomenon.