Model Atmosphere Analysis of Two Very Cool White Dwarfs

A detailed analysis of the very cool white dwarfs SDSS 1337+00 and LHS 3250is presented. Model atmosphere calculations with improved collision-inducedabsorptions by molecular hydrogen indicate that a pure hydrogen composition canbe ruled out, and that the strong infrared absorption observed in these coolstars is better explained in terms of collisions of H2 with neutral helium. Itis shown that even though the overall shape of the observed energydistributions can be reproduced reasonably well with helium-rich models, thepeak of the energy distribution near 6000 A is always predicted too narrow. Theextreme helium-rich composition inferred for both objects is discussed in thebroader context of the extremely cool white dwarfs reported in various surveys.Comment: Accepted for publication in the Astrophysical Journal; 14 pages 9 figure