%0 Journal Article
%@ 1432-0746
%A Martins, F
%A Simon-Diaz, S
%A Palacios, A
%A Howarth, I
%A Georgy, C
%A Walborn, NR
%A Bouret, J-C
%A Barba, R
%D 2015
%F discovery:1494039
%I EDP SCIENCES S A
%J Astronomy & Astrophysics
%K stars: massive – stars: abundances – stars: evolution – stars: atmospheres
%N A10
%T Surface abundances of ON stars
%U https://discovery-pp.ucl.ac.uk/id/eprint/1494039/
%V 578
%X Context. Massive stars burn hydrogen through the CNO cycle during most of their evolution. When mixing is efficient or when mass  transfer in binary systems occurs, chemically processed material is observed at the surface of O and B stars.  Aims. ON stars show stronger lines of nitrogen than morphologically normal counterparts. Whether this corresponds to the presence  of material processed through the CNO cycle is not known. Our goal is to answer this question.  Methods. We performed a spectroscopic analysis of a sample of ON stars with atmosphere models. We determined the fundamental  parameters as well as the He, C, N, and O surface abundances. We also measured the projected rotational velocities. We compared the  properties of the ON stars to those of normal O stars.  Results. We show that ON stars are usually rich in helium. Their CNO surface abundances are fully consistent with predictions of  nucleosynthesis. ON stars are more chemically evolved and rotate − on average − faster than normal O stars. Evolutionary models  including rotation cannot account for the extreme enrichment observed among ON main sequence stars. Some ON stars are members  of binary systems, but others are single stars as indicated by stable radial velocities. Mass transfer is therefore not a simple explanation  for the observed chemical properties.  Conclusions. We conclude that ON stars show extreme chemical enrichment at their surface, consistent with nucleosynthesis through  the CNO cycle. Its origin is not clear at present.
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