Clump Stars in the Solar Neighbourhood

Hipparcos data has allowed the identification of a large number of clump stars in the Solar Neighbourhood. We discuss our present knowledge about their distributions of masses, ages, colours, magnitudes, and metallicities. We point out that the age distribution of clump stars is “biased” towards intermediate-ages. Therefore, the metallicity information they contain is different from that provided by the local G dwarfs. Since accurate abundance determinations are about to become available, these may provide useful constraints to chemical evolution models of the local disc. 1. The clump after Hipparcos The clump of core-helium burning stars has been known for decades in the colour-magnitude diagrams (CMD) of open clusters and nearby galaxies. However, only recently we have been able to identify a significant number of clump stars in the Solar Neighbourhood, thanks to the Hipparcos mission (Perryman et al. 1997). In fact, the ESA (1997) catalog contains ∼ 600 clump stars with parallax error lower than 10 %, and hence an error in absolute magnitude lower than 0.12 mag. This accuracy limit corresponds to a distance of ∼ 125 pc within which the sample of clump stars is complete. Moreover, accurate BV photometry (and I for ∼ 1/3 of the sample), is available, and interstellar absorption is small enough to be neglected. The Hipparcos MV versus B − V CMD is illustrated on the left panel of Fig. 1. In this plot, we exclude binaries and limit the sample to stars with π < 0.007 arcsec and V < 8.5. The clump is well evident at MV ≃ 1, B − V ≃ 1. So far, most of the emphasis on nearby clump stars has been on their possible use as standard candles, since their mean absolute magnitude can be determined with an accuracy of hundredths of magnitude from Hipparcos database (cf. Paczyński & Stanek 1998; Stanek et al. 1998). This application requires a good understanding of the basic characteristics of clump stars, in