The ON stars: A wider spectroscopic definition

All the stars defined as ON by Walborn have spectral type restricted between O9 and O9.5 for the dwarfs, and at O9.7 for the super giants. The probable explanation of this fact is that the NIII line lambdalambda4634-40 normally used as criterion to define ON star, shows a strong tendency into emission when temperature and luminosity increases. Due to this behaviour there is a wide range in luminosity and temperature when the blend reaches minimum intensity values or completely disappears when tending into emission. In addition the positive luminosity effect shown by NIII lambdalambda4634-40 at the higher temperature is so strong that it becomes very difficult to distinguish any anomaly from the normal luminosity dependence. In an attempt to extend the detection of the nitrogen anomalies to a wider range of spectral types and luminosity classes the authors have carried out a spectroscopic analysis of the NIII line lambda4514. (Auth.)