Fine structure of the red giant clump from Hipparcos data, and distance determinations based on its mean magnitude

The I-band brightness M-I of clump stars is a possible distance indicator f or stellar populations. Investigations have shown that M-I is almost insens itive to the V - I colour within the clump. Based on this, it was assumed t hat M-I was insensitive to age and composition of the stellar population an d therefore an ideal standard candle, which could be calibrated with local clump stars whose absolute brightness is known from Hipparcos parallaxes. T his resulted in a distance to the Large Magellanic cloud about 15 per cent smaller than usually determined. In the present paper we show that with a population synthesis approach we c an reproduce the constancy of M-I with colour for the local Hipparcos clump sample. Nevertheless, M-I is not a constant among different populations, b ut depends on metallicity. As a result, the calculated distance modulus to the LMC of 18.28 +/- 0.14 mag is in better agreement with standard values. This resolves, at least partially, the controversial result obtained by the assumption of a universal value for M-I. Particularly remarkable is our prediction that stars slightly heavier than the maximum mass for developing degenerate He cores, M-Hef, should define a secondary clumpy structure, about 0.3 mag below the bluest extremity of th e red clump. Both features are well separated in the M-I versus V - I diagr am of metal-rich stellar populations. Indeed, this secondary clump can be c learly identified in the Hipparcos data base of stars with reliable I photo metry and parallax errors smaller than 10 per cent. Since the stars in this feature should represent a narrow range of masses, their mass determinatio n, e.g. by the use of binary systems, can provide information about the eff iciency of convective overshooting from stellar cores. Our investigation demonstrates that the red giant branch clump cannot be us ed as a distance indicator without proper knowledge and modelling of the po pulation under investigation. In addition, there remain unsolved problems i n the models, such as correct bolometric corrections and colour transformat ions.