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G. Tautvaiien 1,
B. Edvardsson 2,
I. Tuominen 3 and
I. Ilyin 3
Received 25 March 1999 / Accepted 25 May 2000
High-resolution spectra of six core helium-burning `clump' stars and three giants in the open cluster M 67 have been obtained with the SOFIN spectrograph on the Nordic Optical Telescope to investigate abundances of up to 25 chemical elements. Abundances of carbon were studied using the Swan (0,1) band head at 5635.5 Å. The wavelength interval 7980-8130 Å with strong CN features was analysed in order to determine nitrogen abundances and isotope ratios. The oxygen abundances were determined from the [O I] line at 6300 Å.
The overall metallicity of the cluster stars was found to be close to solar ([Fe/H]=). Compared with the Sun and other dwarf stars of the Galactic disk, as well as with dwarf stars of M 67 itself, abundances in the investigated stars suggest that carbon is depleted by about 0.2 dex, nitrogen is enhanced by about 0.2 dex and oxygen is unaltered. Among other mixing-sensitive chemical elements an overabundance of sodium may be suspected. The mean C/N and ratios are lowered to the values of and in the giants and to the values of and in the clump stars. These results suggest that extra mixing of CN-cycled material to the stellar surface takes place after the He-core flash. Abundances of heavy chemical elements in all nine stars were found to be almost identical and close to solar.
Key words: stars: abundances stars: atmospheres stars: horizontal-branch Galaxy: open clusters and associations: individual: M 67
* Based on observations obtained at the Nordic Optical Telescope, La Palma
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Online publication: August 17, 2000