The Giant Branch of Omega Centauri. I. Abundance Variations due to Mixing
Abstract
In an endeavor to understand the anomalously wide giant branch of w sCentauri we have obtained David Dunlap Observatory (DDO) sintermediate-band and RI photometry, together with low-dispersion sspectra, of a representative sample of stars on the upper giant branch. sAnalysis of these data leads to the following conclusions. i) The large swidth of the giant branch is inseparably connected with mixing. All sstars on the red side of the upper giant branch appear to have greatly senhanced features of the CN molecule, with no comparable enhancement of s[Fe/H]. The same result has been obtained by Dickens and Bell. ii) A spositive correlation between [Fe/H] and the CN excess C(4l - 42) exists sin w Cen similar to that reported by McClure and Norris for NGC 362. We ssuggest that this can be explained by the effect of the strong CN band sat A3800 on the 38 filter of the DDO system. A broad continuum sdepression around A4000, similar to that discovered by Bond and Neff in sthe Ba ii and CH stars, exists in the mixed stars and may also scontribute to the correlation. iii) The stars on the blue side of the sgiant branch show no evidence for mixing and yield an abundance [Fe/H] = s-2.1 + 0.2. Therefore while the existence of mixing in the redder stars sprevents us from addressing the problem of a possible range in abundance swhich may have existed at the end of the epoch of star formation in w sCen (as has been suggested by Freeman and Rodgers) it appears that the smaterial from which the cluster formed was as metal deficient as the svery metal poor globular clusters. iv) The strong CN enhancement in sstars on the red side of the giant branch is not accompanied by greatly senhanced features of CH and C2 as found in the CH stars. We suggest that sthe CN stars have 0/C > 1 and that during the mixing process much of sthe material now seen at the surface of these objects has been processed sthrough the CN cycle. v) The large width of the giant branch seen in the s(V, B - V)-plane is greatly reduced in the (R, R - 1)-plane. This ssuggests to us that blocking effects are the predominant phenomenon scausing the observed spread in (B - V). In conclusion, we consider the sproblem that w Cen is apparently unique in possessing an anomalously swide giant branch. We investigate the possibility that the effect could sresult from anomalously large angular momentum, and suggest that it smight be profitable to observe the highly flattened cluster NGC 6273 to sascertain if it exhibits the same phenomenon. Subject headings: sclusters: globular - convection - stars: abundance stars: evolution - sstars: interiors - stars: late-type
- Publication:
-
The Astrophysical Journal
- Pub Date:
- September 1976
- DOI:
- 10.1086/154616
- Bibcode:
- 1976ApJ...208..369B