Astron. Astrophys. 360, 499-508 (2000)

1. Introduction

The old open cluster M 67 has served as an important sample in the understanding of stellar evolution over almost fifty years. Since the first papers by Becker & Stock (1952), Popper (1954), Johnson & Sandage (1955) work has continued and more than 200 studies were achieved (see, e.g., Burstein et al. 1986, Carraro et al. 1996 and references therein). The advantage that cluster members have to be coeval and identical except for mass and evolutionary state, which can be identified unambiguously, may most efficiently serve for the analysis of changes in mixing-sensitive abundances.

Abundances of carbon and nitrogen are particularly sensitive tests for stellar evolution. The enhancement of CN bands was reported for the M 67 clump stars F84, F141 and F151 already by Pagel (1974). From high-resolution spectra, C and N abundances have been investigated in three giants (Brown 1985) and from moderate-resolution spectra in 19 giants (Brown 1987). Carbon isotope ratios along the giant branch were investigated by Gilroy (1989) and Gilroy & Brown (1991). It was found that C/N and ratios in the clump giants and the stars at the tip of the giant branch all have values much lower than predicted in standard models. Charbonnel et al. (1998), however, suggest that the absolute values of C/N ratios obtained by Brown (1987) may have a systematic offset and have to be taken with caution.

High-resolution analyses are very scarce for the oxygen abundances: Griffin (1975) has measured the [O I] line at 6300 Å in one star, but this star (IV-202) is quite cool and the result is uncertain; Cohen (1980) has analysed four stars, but the weaker line of [O I] at 6363 Å was used. In the paper by Cohen (1980), a low value of [Fe/H] for M 67 has been received. This is the same paper that gave a low [Fe/H] value for the globular cluster M 71, which was later increased by +0.5 dex in Cohen (1983). The same correction, if applied to M 67, would yield the [Fe/H] near solar, consistent with more recent determinations (e.g. Nissen et al. 1987, Garcia Lopez et al. 1988, Hobbs & Thorburn 1991, Friel & Boesgaard 1992). In the same paper by Cohen (1980), which is based on photographic data as well as all M 67 abundance works before Brown (1985), abundances of some other elements in M 67 look very extraordinary. The ratios of [Mg/Fe] reach -0.8 dex while the ratios of other -process element [Si/Fe] are enhanced by about +0.6 dex, [Ba/Fe] are approximately equal to -0.4 dex while for the very similar element lanthanum [La/Fe]+0.6 dex.

In this paper, we report a detailed analysis of six core helium-burning clump stars and three giants in M 67 (see Fig. 1 for their location in the HR diagram). The core He-burning stars are the most evolved stars in M 67, their surface abundances reflect effects of the preceding evolution along the red giant branch as well as effects raised by the helium flash. The study aims at a very high internal precision of the abundances, so that even small anomalies in the chemical composition can be revealed.

Fig. 1. The colour-magnitude diagram of the open cluster M 67. The red giants and the core He-burning `clump' stars analysed are indicated by the crosses. The diagram is taken from Mathieu et al. (1986) and cleaned from evident binary stars

© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000
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