Lyman-Line Decrements in Low-Density Planetary Nebulae

The radiation fields of Lyman-a to Lyman-lO have been calculated for two model planetary nebulae having constant and exponential density distributions in spherical symmetry. The decrement for the higher lines is very flat. Both the decrement and the photon density distribution of the lines are shown to be rather insensitive to the density distribution. An attempt is made to understand these results in terms of elementary arguments. Observations of the Lyman lines in planetary nebulae will provide in- formation about neutral gas at high galactic latitudes and determine the integrated photon luminosity of the central star in the Lyman continuum