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

Abstract

The I-band brightness M_I of clump stars is a possible distance indicator for stellar populations. Investigations have shown that M_I is almost insensitive to the V-I colour within the clump. Based on this, it was assumed that M_I was insensitive to age and composition of the stellar population and therefore an ideal standard candle, which could be calibrated with local clump stars whose absolute brightness is known from Hipparcos parallaxes. This 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 can reproduce the constancy of M_I with colour for the local Hipparcos clump sample. Nevertheless, M_I is not a constant among different populations, but 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 the red clump. Both features are well separated in the M_I versus V-I diagram of metal-rich stellar populations. Indeed, this secondary clump can be clearly identified in the Hipparcos data base of stars with reliable I photometry and parallax errors smaller than 10 per cent. Since the stars in this feature should represent a narrow range of masses, their mass determination, e.g. by the use of binary systems, can provide information about the efficiency of convective overshooting from stellar cores. Our investigation demonstrates that the red giant branch clump cannot be used as a distance indicator without proper knowledge and modelling of the population under investigation. In addition, there remain unsolved problems in the models, such as correct bolometric corrections and colour transformations.