Model Atmosphere Analysis of Two Very Cool White Dwarfs

A detailed analysis of the very cool white dwarfs SDSS 1337+00 and LHS 3250 is presented. Model atmosphere calculations with improved collision-induced absorptions by molecular hydrogen indicate that a pure hydrogen composition can be ruled out, and that the strong infrared absorption observed in these cool stars is better explained in terms of collisions of H 2 with neutral helium. It is shown that even though the overall shape of the observed energy distributions can be reproduced reasonably well with helium-rich models, the peak of the energy distribution near 6000Å is always predicted too narrow. The extreme helium-rich composition inferred for both objects is discussed in the broader context of the extremely cool white dwarfs reported in various surveys.