The far-UV spectrum of the low-excitation planetary nebula HD 138403

Two high-resolution far-UV spectra of the low-excitation planetary nebula HD 138403 are analyzed which were obtained with the IUE satellite over the wavelength range from 1170 to 2070 A. It is shown that the nebula's far-UV spectrum comprises a stellar continuum on which are superimposed a few emission lines, numerous interstellar absorption lines, and various types of P Cygni profiles. Evidence is examined for substantial mass loss from the central nucleus, with terminal velocities of the order of 800 km/s. The observed nebular emissions are used to derive the electron density in the main nebula. The results are taken as supporting the hypothesis that a bipolar structure is expanding around HD 138403 via a selective radiation process involving an edge- or line-locking mechanism or both.