The Trace of the CNO Cycle in the Ring Nebula NGC6888
  We present new results on the chemical composition of the Galactic ring
nebula NGC6888 surrounding the WN6(h) star WR136. The data are based on deep
spectroscopical observations taken with the High Dispersion Spectrograph at the
8.2m Subaru Telescope. The spectra cover the optical range from 3700 to 7400 A.
The effect of the CNO cycle is well identified in the abundances of He, N, and
O, while elements not involved in the synthesis such as Ar, S, and Fe present
values consistent with the solar vicinity and the ambient gas. The major
achievement of this work is the first detection of the faint CII 4267
recombination line in a Wolf-Rayet nebula. This allows to estimate the C
abundance in NGC6888 and therefore investigate for the first time the trace of
the CNO cycle in a ring nebula around a Wolf-Rayet star. Although the detection
of the CII line has a low signal-to-noise ratio, the C abundance seems to be
higher than the predictions of recent stellar evolution models of massive
stars. The Ne abundance also show a puzzling pattern with an abundance of about
0.5 dex lower than the solar vicinity, which may be related to the action of
the NeNa cycle. Attending to the constraints imposed by the dynamical timescale
and the He/H and N/O ratios of the nebula, the comparison with stellar
evolution models indicates that the initial mass of the stellar progenitor of
NGC6888 is between 25 Msun and 40 Msun.
