The giant branch of omega Centauri. I. Abundance variations due to mixing

Journal Article · Wed Sep 01 04:00:00 EDT 1976 · Astrophys. J.; (United States)

DOI:https://doi.org/10.1086/154616· OSTI ID:7261764

Bessell, M S; Norris, J

David Dunlap Observatory (DDO) intermediate-band and RI photometry, together with low-dispersion spectra of a representative sample of stars on the upper giant branch were analysed. Several conclusions were: i) The large width of the giant branch is inseparably connected with mixing. All stars on the red side of the upper giant branch appear to have greatly enhanced features of the CN molecule, with no comparable enhancement of (Fe/H). ii) A positive correlation between (Fe/H) and the CN excess deltaC (41--42) exists in ..omega.. Cen similar to that reported by McClure and Norris for NGC 362. We suggest that this can be explained by the effect of the strong CN band at lambda3800 on the 38 filter of the DDO system. A broad continuum depression around lambda4000 exists in the mixed stars and may also contribute to the correlation. iii) The stars on the blue side of the giant branch show no evidence for mixing and yield an abundance (Fe/H)=-2.1 +- 0.2. It appears that the material from which the cluster formed was as metal deficient as the very metal poor globular clusters. iv) The strong CN enhancement in stars on the red side of the giant branch is not accompanied by greatly enhanced features of CH and C/sub 2/ as found in the CH stars. We suggest that the CN stars have O/C>1 and that during the mixing process much of the material now seen at the surface of these objects has been processed through the CN cycle. v) The large width of the branch seen in the (V, B--V) -plane is greatly reduced in the (R, R--I) -plane. This suggests to us that blocking effects are predominant in causing the observed spread in (B--V). We consider the problem that ..omega.. Cen is apparently unique in possessing an anomalously wide giant branch. We investigate the possibility that the effect could result from anomalously large angular momentum, and suggest that it might be profitable to observe the highly flattened cluster NGC 6273 to ascertain if it exhibits the same phenomenon. (AIP)

• Research Organization: Mount Stromlo and Siding Spring Observatory, Research School of Physical Sciences, Australian National University
• OSTI ID: 7261764
• Journal Information: Astrophys. J.; (United States), Journal Name: Astrophys. J.; (United States) Vol. 208:2; ISSN ASJOA
• Country of Publication: United States
• Language: English

Related Subjects

640105* -- Astrophysics & Cosmology-- Galaxies
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
ABUNDANCE
CHEMICAL COMPOSITION
CYANOGEN
GIANT STARS
MIXING RATIO
PHOTOMETRY
RED GIANT STARS
STAR CLUSTERS
STAR EVOLUTION
STARS