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What's the proportion of oxygen in a typical white dwarf relative to the proportion of Carbon?

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From my answer to a recent question on crystallization in white dwarfs I point to Horowitz et al. with a nice Physical Review Letter on "Crystallization of Carbon-Oxygen Mixtures in White Dwarf Stars". Part of their paper is trying to constrain the oxygen content in order to constrain the cross sections for the C($\alpha, \gamma$)O reaction. Their answer is not exactly precise. They state in the abstract:

Observations of WD crystallization in the globular cluster NGC 6397 by Winget et al. suggest that the melting temperature of WD cores is close to that for pure carbon. If this is true, our phase diagram implies that the central oxygen abundance in these stars is less than about 60%.

Figure 2 in the paper shows the various calculated melting temperatures vs composition from their work and other literature, and only above 60% is where the melting temperature must exceed close to the melt temperature of pure carbon. It could be lower, depending on which simulation is closer to reality.

So, the atomic concentration of oxygen is below 60%.

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