New Luminous ON Spectra from the Galactic O-Star Spectroscopic Survey

TL;DR

This paper reports the discovery of new ON and ONn supergiant and giant stars from the Galactic O-Star Spectroscopic Survey, highlighting their spectral diversity, physical properties, and implications for stellar evolution and mixing processes.

Contribution

The study presents the identification and analysis of seven ON supergiants and eight ONn giants, expanding the known sample and exploring their spectral characteristics and physical implications.

Findings

Seven new ON supergiants identified, expanding the class.

Eight ONn giants characterized by similar spectra, differing mainly in line broadening.

Spectral variability observed in a hypergiant star.

Abstract

Two new ON supergiant spectra (bringing the total known to seven) and one new ONn giant (total of this class now eight) are presented; they have been discovered by the Galactic O-Star Spectroscopic Survey. These rare objects represent extremes in the mixing of CNO-cycled material to the surfaces of evolved, late-O stars, by uncertain mechanisms in the first category but likely by rotation in the second. The two supergiants are at the hot edge of the class, which is a selection effect from the behavior of defining N III and C III absorption blends, related to the tendency toward emission (Of effect) in the former. An additional N/C criterion first proposed by Bisiacchi et al. is discussed as a means to alleviate that effect, and it is relevant to the two new objects. The entire ON supergiant class is discussed; they display a fascinating diversity of detail undoubtedly related to the complexities of their extended atmospheres and winds that are sensitive to small differences in physical parameters, as well as to binary effects in some cases. Serendipitously, we have found significant variability in the spectrum of a little-known hypergiant with normal N, C spectra selected as a comparison for the anomalous objects. In contrast to the supergiants, the ONn spectra are virtual (nitrogen)-carbon copies of one another except for the degrees of line broadening, which emphasizes their probable unique origin and hence amenability to definitive astrophysical interpretation.