  Abundances relative to iron for carbon, nitrogen, strontium and barium are
presented for 33 stars on the red giant branch of the globular cluster omega
Centauri. They are based on intermediate-resolution spectroscopic data covering
the blue spectral region analyzed using spectrum synthesis techniques. The data
reveal the existence of a broad range in the abundances of these elements, and
a comparison with similar data for main sequence stars enables insight into the
evolutionary history of the cluster. The majority of the red giant branch stars
were found to be depleted in carbon, i.e. [C/Fe]<0, while [N/Fe] for the same
stars shows a range of ~1 dex, from [N/Fe]~0.7 to 1.7 dex. The
strontium-to-iron abundance ratios varied from solar to mildly enhanced
(0.0<=[Sr/Fe]<=0.8), with [Ba/Fe] generally equal to or greater than [Sr/Fe].
The carbon and nitrogen abundance ratios for the one known CH star in the
sample, ROA 279, are [C/Fe]=0.6 and [N/Fe]=0.5 dex. Evidence for evolutionary
mixing on the red giant branch is found from the fact that the relative carbon
abundances on the main sequence are generally higher than those on the red
giant branch. However, comparison of the red giant branch and main sequence
samples shows that the upper level of nitrogen enhancement is similar in both
sets at [N/Fe]~2.0dex. This is most likely the result of primordial rather than
evolutionary mixing processes. One red giant branch star, ROA 276, was found to
have Sr and Ba abundance ratios. High resolution spectra of ROA 276 were
obtained with the Magellan Telescope/MIKE spectrograph combination to confirm
this result, revealing that ROA 276 is indeed an unusual star. For this star
calculations of the depletion effect strongly suggest that the observed Sr
enhancement in ROA 276 is of primordial origin, rather than originating from a
surface accretion event.
