Can the EXOSAT Observations of DA White Dwarfs Be Explained by Layered Atmospheric Structures?

Abstract

It is demonstrated that the Exosat broadband filter observations of DA white dwarfs, which have recently been interpreted with homogeneously mixed H/He atmospheres (Jordan et al, 1987; Paerels and Heise, 1989), can be explained with stratified theoretical models. In these models a very thin H layer is floating on top of the underlying He. The transition zone is determined by diffusive equilibrium. The resulting total hydrogen-layer masses are in the range (3-500) x 10 to the -16th solar mass. Several arguments are discussed that could lead to a decision about the correct interpretation, but a final answer is not possible.