Detection of Lyman alpha in the spectrum of a white dwarf with heliumatmosphere.

Abstract

The detection of Ly-alpha absorption in a low dispersion IUE spectrum of the DBA white dwarf G200-39 is reported. The geocoronal emission was reduced by using a small aperture. For effective temperatures between 12,000 and 16,000, and log n(H)/n(He) values of -5, -4.5, -4, and -3.5, calculations giving colors and line strengths for He I 4472 A, H-alpha, H-beta, and H-gamma are presented. The results are compared with the G200-39 Ly-alpha profiles, showing an effective temperature of 14,500 + or - 500 K and a long n(H)/n(He) value of -4.0 + or - 0.25. It is shown that Ly-alpha is a strong indicator of trace hydrogen in DB star atmospheres.