G74-7 - A true DA,F (DAZ) white dwarf

Abstract

The first results of an observational attempt to confirm the existence of DA,F white dwarfs (cool, hydrogen-rich with calcium lines) using precision digital spectrophotometry are reported. High-resolution observations of 16 cool white dwarfs (Te less than or approximately 9500 K), variously classified as DA, DA,F, or DF stars were performed. The survey yielded only one object, G74-7, with a sharp and narrow Ca II K line of equivalent width 0.7 A. Upper limits on the width of the K line in the other objects are typically about 0.25 A. The corresponding photospheric calcium abundance in G74-7 is Ca/H of about 1.2 billionths, and the line profile is consistent with theoretical predictions. The origin of the observed calcium is discussed and it is shown that the heavy element diffusion time scale in the envelope is short compared to the evolutionary time scale of the white dwarf. It is suggested that the observed calcium in G74-7 is the signature of a recent encounter with an interstellar cloud of moderate density.