Can Pure Helium White Dwarfs Support a Habitable Zone for Billions of Years?

[qraal]

Just pondering Eric Agol's recent paper on White Dwarf habitable zones and the pace of cooling of white dwarfs (WDs.) He uses models of Carbon core WDs with helium atmospheres, but I am wondering if pure helium WDs would last three times longer. Opacity from the atmosphere slows down heat loss significantly else WDs would cool very quickly since the interior is nearly isothermal from highly efficient heat conduction. Thus there should be three times the heat energy available since helium masses one third carbon and energy of a gas depends on the numbers of atoms, not the whole mass.

In theory higher opacity in Red Giant stars should blow off enough mass before the Red Giant Tip and the Helium flash, so that the Helium Main Sequence is never initiated. Thus a pure helium WD should be possible, given sufficient mass-loss by its Main Sequence precursor. So instead of a maximum CHZ lifetime of 8 Gyr, there should be a 24 Gyr CHZ lifetime for a well placed planet.

 

[lippyka]

However, the actual lifetime of a pure helium WD is likely to be much shorter than this due to the lower opacity of the atmosphere. Pure helium WDs are expected to cool faster than Carbon core WDs due to their lack of significant carbon opacity. This could reduce the CHZ lifetime by a factor of 3-5 relative to a Carbon core WD. Thus the maximum lifetime of a pure Helium WD CHZ could be around 5-8 Gyr. In addition, the actual lifetimes of pure helium WDs may be further reduced since they are expected to suffer from a form of thermal instabilities known as the 'double degenerate instability', which can cause rapid and extreme cooling. This could limit the lifetime of a pure helium WD CHZ to just a few billion years.