A secondary clump of red giant stars: why and where

Based on the results of detailed population synthesis models, Girardi et al . recently claimed that the clump of red giants in the colour-magnitude dia gram (CMD) of composite stellar populations should present an extension to lower luminosities, which goes down to about 0.4 mag below the main clump. This feature is made of stars just massive enough to have ignited helium in non-degenerate conditions, and therefore corresponds to a limited interval of stellar masses and ages. In the present models, which include moderate convective overshooting, it corresponds to similar to 1 Gyr old populations . In this paper, we go into more detail about the origin and properties of th is feature. We first compare the clump theoretical models with data for clu sters of different ages and metallicities, basically confirming the predict ed behaviour. We then refine the previous models in order to show the follo wing behaviour. (i) The faint extension is expected to be clearly separated from the main clump in the CMD of metal-rich populations, defining a 'seco ndary clump' by itself. (ii) It should be present in all galactic fields co ntaining similar to 1 Gyr old stars and with mean metallicities higher than about Z = 0.004. (iii) It should be particularly strong, if compared with the main red clump, in galaxies that have increased their star formation ra te in the last Gyr or so of their evolution. In fact, secondary clumps simi lar to the model predictions are observed in the CMD of nearby stars from H ipparcos data, and in those of some Large Magellanic Cloud fields observed to date. There are also several reasons why this secondary clump may be mis sing or hidden in other observed CMDs of galaxy fields. For instance, it be comes indistinguishable from the main clump if the photometric errors or di fferential absorption are larger than about 0.2 mag. None the less, this st ructure may provide important constraints on the star formation history of Local Group galaxies. We comment also on the intrinsic luminosity variation and dispersion of clump stars, which may limit their use as either absolut e or relative distance indicators, respectively.