On the peculiar red clump morphology in the open clusters NGC 752 and NGC 7789

Abstract

The red clump stars in the open cluster NGC 752 present a peculiar distribution in the colour-magnitude diagran (CMD): the clump is observed to present a faint extension, slightly to the blue of the main concentration of clump stars. We point out that a similar structure is present in the CMD of NGC 7789, and discuss their possible origins. This feature may be understood as the result of having, at the same time, stars of low-mass which undergo the helium-flash, and those just massive enough for avoiding it. The ages of both clusters are compatible with this interpretation. Similar features can be produced in theoretical models which assume a non-negligible mass spread for clump stars, of about 0.2 Mo. However, one can probably exclude that the observed effect is due to the natural mass range of core helium burning stars found in single isochrones, although present models do not present the level of detail necessary to completely explore this possibility. Also the possibility of a large age spread among cluster stars can be refuted on observational grounds. We then suggest a few alternatives. This spread may be resulting either from star-to-star variations in the mass-loss rates during the RGB phase. Alternatively, effects sush as stellar rotation or convective core overshooting, could be causing a significant spread in the core mass at He-ignition for star of similar mass. Finally, we point out that similar effects could also help to understand the distribution of clump stars in the CMDs of the clusters NGC 2660 and NGC 2204.