Far and extreme-ultraviolet spectrophotometry of hot white dwarfs : prospects for the determination of the He/H ratio in DA stars.

Abstract

The various spectroscopic methods currently available to determine the He/H ratio in hot DA white dwarfs are compared and contrasted. The observation of the optical He II λ4686 line, as well as that of the λ1640 line with the low-dispersion IUE camera, can only be wsed to determine the helium abundance for log (He/H) > -3. Low-resolution (λ/Δλ ∼ 20) rocket-borne extreme ultraviolet spectrophotometry, such as that performed on HZ 43, is more sensitive to the abundance ratio, but only through the strength of the He II edge at 227 λ. We contrast these results with those expected from the Faint Object Spectrograph on the Space Telescope and from the Far Ultraviolet Spectroscopic Explorer. We show, in particular, that medium-resolution (λ/Δλ ∼ 100) extreme ultraviolet spectrophotometry remains sensitive to the helium abundance-through the He II resonance series-down to trace levels of log (He/H) ≈ -8. Measurements of hot DA white dwarfs in that wavelength range are thus expected to enhance considerably our understanding of the efficiency of element diffusion, and of competing processes, in the photospheres of these stars.