Further Results from the Galactic O-Star Spectroscopic Survey: Rapidly Rotating Late ON Giants

TL;DR

This paper expands the classification of rapidly rotating, nitrogen-enriched late-O giants, revealing their unique properties and potential origins, including binary interactions and high initial rotation, through spectroscopic survey data.

Contribution

It identifies and characterizes new rapidly rotating ONn giants, highlighting their evolutionary significance and possible formation mechanisms.

Findings

All ON giants are rapid rotators, unlike dwarfs and supergiants.

ONn giants show high rotation and processed material, indicating intermediate evolutionary stage.

Some ONn giants are runaway stars, suggesting diverse origins.

Abstract

With new data from the Galactic O-Star Spectroscopic Survey, we confirm and expand the ONn category of late-O, nitrogen-enriched (N), rapidly rotating (n) giants. In particular, we have discovered two "clones" (HD 102415 and HD 117490) of one of the most rapidly rotating O stars previously known (HD 191423, "Howarth's Star"). We compare the locations of these objects in the theoretical HR Diagram to those of slowly rotating ON dwarfs and supergiants. All ON giants known to date are rapid rotators, whereas no ON dwarf or supergiant is; but all ON stars are small fractions of their respective spectral-type/luminosity-class/rotational subcategories. The ONn giants, displaying both substantial processed material and high rotation at an intermediate evolutionary stage, may provide significant information about the development of those properties. They may have preserved high initial rotational velocities or been spun up by TAMS core contraction; but alternatively and perhaps more likely, they may be products of binary mass transfer. At least some of them are also runaway stars.