The ages and colours of cool helium-core white dwarf stars (original) (raw)

The purpose of this work is to explore the evolution of helium-core white dwarf stars in a selfconsistent way with the predictions of detailed non-grey model atmospheres and element diffusion. To this end, we consider helium-core white dwarf models with stellar masses of 0.406, 0.360, 0.327, 0.292, 0.242, 0.196 and 0.169 M (and follow their evolution from the end of mass-loss episodes, during their pre-white dwarf evolution, down to very low surface luminosities. We find that when the effective temperature decreases below 4000 K, the emergent spectrum of these stars becomes bluer within timescales of astrophysical interest. In particular, we analyse the evolution of our models in the colour-colour and in the colourmagnitude diagrams and find that helium-core white dwarfs with masses ranging from ,0.18 to 0.3 M (can reach the turn-off in their colours and become blue again within cooling times much less than 15 Gyr and then remain brighter than M V < 16:5. In view of these results, many low-mass helium white dwarfs could have had enough time to evolve to the domain of collision-induced absorption from molecular hydrogen, showing blue colours.