Evidence for the stratification of Fe in the photosphere of G191-B2B

Abstract

The presence of heavy elements in the atmospheres of the hottest H-rich DA white dwarfs has been the subject of considerable interest. While theoretical calculations can demonstrate that radiative forces, counteracting the effects of gravitational settling, can explain the detections of individual species, the predicted abundances do not accord well with observation. However, accurate abundance measurements can only be based on a thorough understanding of the physical structure of the white dwarf photospheres, which has proved elusive. Recently, the availability of new non-LTE model atmospheres with improved atomic data has allowed self-consistent analysis of the EUV, far UV and optical spectra of the prototypical object G191$-$B2B. Even so, the predicted and observed stellar fluxes remain in serious disagreement at the shortest wavelengths (below $\approx 190$Å), while the inferred abundances remain largely unaltered. We show here that the complete spectrum of G191$-$B2B can be explained by a model atmosphere where Fe is stratified, with increasing abundance at greater depth. This abundance profile may explain the difficulties in matching observed photospheric abundances, usually obtained by analyses using homogeneous model atmospheres, to the detailed radiative levitation predictions. Particularly as the latter are only strictly valid for regions deeper than where the EUV/far UV lines and continua are formed. Furthermore, the relative depletion of Fe in the outer layers of the atmosphere may be evidence for radiatively driven mass loss in G191$-$B2B.

12 pages, 7 Figures, accepted for publication in MNRAS

Cited by in corpus (14)

• Evidence for an external origin of heavy elements in hot DA white dwarfs
• Heavy element abundance patterns in hot DA white dwarfs
• A comparison of DA white dwarf temperatures and gravities from FUSE Lyman line and ground-based Balmer line observations
• Circumstellar features in hot DA white dwarfs
• Absolute Flux Calibration of the IRAC Instrument on the Spitzer Space Telescope using Hubble Space Telescope Flux Standards
• Equilibrium abundances in hot DA white dwarfs as derived from self-consistent diffusion models I. Analysis of spectroscopic EUVE data
• Measuring the fine structure constant on a white dwarf surface; a detailed analysis of Fe V absorption in G191-B2B
• A comparison of DA white dwarf temperatures and gravities from Lyman and Balmer line studies
• High-Resolution Spectroscopy of G191-B2B in the Extreme Ultraviolet
• The discovery of photospheric nickel in the hot DO white dwarf REJ0503-289
• Heavy element abundances in DAO white dwarfs measured from FUSE data
• A comprehensive near and far ultraviolet spectroscopic study of the hot DA white dwarf G191-B2B
• Constraining the magnetic field on white dwarf surfaces; Zeeman effects and fine structure constant variation
• A photospheric metal line profile analysis of hot DA white dwarfs with circumstellar material