The ages and colours of cool helium-core white dwarf stars |
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| Authors: | AM Serenelli † LG Althaus ‡ RD Rohrmann † OG Benvenuto § |
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| Institution: | ;1Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque S/N, (1900) La Plata, Argentina;2Observatorio Astronómico, Universidad Nacional de Córdoba, Laprida 854, (5000) Córdoba, Argentina |
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| Abstract: | The purpose of this work is to explore the evolution of helium-core white dwarf stars in a self-consistent 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 time-scales of astrophysical interest. In particular, we analyse the evolution of our models in the colour–colour and in the colour–magnitude 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. |
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| Keywords: | stars: atmospheres stars: evolution stars: fundamental parameters stars: interiors white dwarfs |
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