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

Abstract

The I-band brightness of clump stars has been used recently to determine the distance to the bulge, the LMC and SMC, and to M31. It was assumed to be a standard candle based on the observational fact that it is independent of (V-I)-colour in all observed populations. The calibration was done using nearby clump stars with HIPPARCOS-parallaxes. The most surprising result of this procedure was the claim that the distance to the LMC was only 18.04 mag. 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 determined distance modulus to the LMC of 18.28+-0.18 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. We also find and discuss substructures in the clump: we predict that stars slightly heavier than the maximum mass for developing degenerate He cores should define a secondary, clumpy structure, about 0.3 mag below the bluest extremity of the red clump. This secondary clump can be clearly identified in the HIPPARCOS database of stars. We argue that there remain severe uncertainties in the use of clump stars as a distance indicator.