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.
Highlights
Massive stars burn hydrogen to helium on the CNO cycle(s); the slowest reactions have the effect of leaving most of these elements as nitrogen, at the outset and for their duration
A key spectral segment in the full current membership of the ONn class is presented in Figure 1, with the prototype subgiant ON spectrum of HD 201345 for comparison (Walborn 1970; its spectral type has been refined from the original in the GOSSS data)
As shown by the further comparison object HD 96264, an O9.5 III standard, in morphologically normal spectra the C III feature is far stronger than the N III. (In OC spectra it is even more so.)
Summary
Massive stars burn hydrogen to helium on the CNO cycle(s); the slowest reactions have the effect of leaving most of these elements as nitrogen, at the outset and for their duration. If this partially (since the conversion of H to He proceeds on the nuclear timescale) processed material is mixed to the surface before the reactions run to completion, enhanced nitrogen to carbon and oxygen abundance ratios become visible. On it was realized that Wolf-Rayet stars of the nitrogen sequence (WN) are revealing the products of H-burning, while the carbon sequence (WC) is more evolved and shows He-burning products (see Crowther 2007 for a recent review and references). It has been shown that the timescale and degree of these effects are functions of the initial stellar rotational velocity (Maeder & Meynet 2000; Heger & Langer 2000)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
