TY  - JOUR
VL  - 578
N1  - This version is the version of record. For information on re-use, please refer to the publisher?s terms and conditions.
IS  - A10
SN  - 1432-0746
UR  - http://doi.org/10.1051/0004-6361/201526130
A1  - Martins, F
A1  - Simon-Diaz, S
A1  - Palacios, A
A1  - Howarth, I
A1  - Georgy, C
A1  - Walborn, NR
A1  - Bouret, J-C
A1  - Barba, R
JF  - Astronomy & Astrophysics
EP  - 16
AV  - public
Y1  - 2015/06/01/
TI  - Surface abundances of ON stars
PB  - EDP SCIENCES S A
ID  - discovery1494039
N2  - 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.
KW  - stars: massive ? stars: abundances ? stars: evolution ? stars: atmospheres
ER  -