A CNO Dichotomy among O2 Giant Spectra in the Magellanic Clouds

Abstract

From a survey of the 3400 {Å} region in the earliest O-type spectra,we have found that two of the four O2 giants observed in the LargeMagellanic Cloud have O IV lines there that are stronger than the N IVlines, while the other two have the opposite. A Small Magellanic Cloudcounterpart also has N IV stronger than O IV. Inspection of the bluespectra of these stars shows that the former pair have weaker N lines inall ionization states (III, IV, and V) present as well as lines of C IV{$λ$}4658, while the latter three have stronger N lines and greaterHe/H. Space ultraviolet observations of two of the N-strong stars show NV wind profiles substantially stronger than those of C IV, while in theN-weak stars the C IV features are equal to or stronger than the N V.The N-strong stars are now reclassified as ON2 III(f*), newly definingthat category. These characteristics strongly suggest a larger fractionof processed material in the atmospheres of the ON2 stars, which weconfirm by modeling the optical spectra. In the context of currentmodels, it is in turn implied that the ON2 stars are in a more advancedevolutionary state than the others, and/or that they had higher initialrotational velocities. The recent formulation of the effects of rotationon massive stellar evolution introduces an additional fundamentalparameter, which the CNO abundances are in principle able to constrain.We present some illustrative comparisons with current Genevaevolutionary models for rotating massive stars. It is possible thatthese very hot, nitrogen-rich objects are products of homogeneousevolution. Our results will provide motivation for further physicalmodeling of the atmospheres and evolutionary histories of the mostmassive hot stars.