The Ultraviolet Spectra of the Weak Emission Line Central Stars of Planetary Nebulae

Abstract

The UV spectra of all "weak emission line central stars of planetary nebulae" (WELSs) with available IUE data are presented and discussed. We performed line identifications and equivalent-width and flux measurements for several features in the spectra. We found that the WELSs can be divided into three different groups regarding their UV spectra: (1) strong P Cygni profiles (mainly in C IV λ1549), (2) weak P Cygni features, and (3) absence of P Cygni profiles. The last group encompasses stars with a featureless UV spectrum or with intense emission lines and a weak continuum, which are most likely of nebular origin. We have measured wind terminal velocities for all objects presenting P Cygni profiles in N V λ1238 and/or C IV λ1549. The results obtained were compared to the UV data of the two prototype stars of the [WC]-PG 1159 class, namely, A30 and A78. For WELSs presenting P Cygni features, most of the terminal velocities fall in the range of ~1000-1500 km s^-1, while [WC]-PG 1159 stars possess much higher values, of ~3000 km s^-1. The [WC]-PG 1159 stars are characterized by intense, simultaneous P Cygni emissions in the ~1150-2000 Å interval of N V λ1238, O V λ1371, and C IV λ1549. In contrast, we found that O V λ1371 is very weak or absent in the WELS spectra. On the basis of the UV spectra alone, our findings indicate that [WC]-PG 1159 stars are distinct from the WELSs, contrary to previous claims in the literature.