Photospheric phosphorus in the FUSE spectra of GD71 and two similar DA white dwarfs

Abstract

We report the detection, from FUSE data, of phosphorus in the atmospheres of GD71 and two similar DA white dwarfs. This is the first detection of a trace metal in the photosphere of the spectrophotometric standard star GD71. Collectively, these objects represent the coolest DA white dwarfs in which photospheric phosphorus has been observed. We use a grid of homogeneous non-LTE synthetic spectra to measure abundances of[P/H]=-8.57 +0.09 -0.13, -8.70 +0.23 -0.37 and -8.36 +0.14 -0.19 in GD71, RE J1918+595 and RE J0605-482 respectively. At the observed level we find phosphorus has no significant impact on the overall energy distribution of GD71. We explore possible mechanisms responsible for the presence of this element in these stars, concluding that the most likely is an interplay between radiative levitation and gravitational settling but possibly modified by weak mass loss.