Spectral analysis of ultra-cool white dwarfs polluted by planetary debris

Understand

We identify two ultra-cool ($T_\mathrm{eff} < 4000$ K) metal-polluted (DZ) white dwarfs WDJ2147$-$4035 and WDJ1922$+$0233 as the coolest and second coolest DZ stars known to date with $T_\mathrm{eff} \approx 3050$ K and $T_\mathrm{eff} \approx 3340$ K, respectively.

• Strong atmospheric collision-induced absorption (CIA) causes the suppression of red optical and infra-red flux in WDJ1922$+$0233, resulting in an unusually blue colour given its low temperature.
• WDJ2147$-$4035 has moderate infra-red CIA yet has the reddest optical colours known for a DZ white dwarf.
• Microphysics improvements to the non-ideal effects and CIA opacities in our model atmosphere code yields reasonable solutions to observations of these ultra-cool stars.