Abstract

The ONn Giants

Nolan R. Walborn (Space Telescope Science Institute)

Jesús Maíz Apellániz (Instituto de Astrofísica de Andalucía)

With new data from the Galactic O-Star Spectroscopic Survey, we expand and enhance the ONn category of late-O, nitrogen-enriched (N), rapidly rotating (n) giants. In particular, we have discovered two clones of the most rapidly rotating O star 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 ON/OC supergiants, and to those of morphologically normal types of the same two-dimensional spectral classes. In fact, all ON giants known to date are rapid rotators, whereas no ON dwarf or supergiant is. These interesting objects, displaying both strong mixing of 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 they may have been spun up by TAMS core contraction or binary mass transfer.