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

The ultraviolet spectra of all ``weak emission line central stars of planetary nebulae (WELS) with available IUE data are presented and discussed. We performed line identifications, equivalent width and flux measurements for several features in their spectra. We found that the WELS can be divided in three different groups regarding their UV: i) Strong P-Cygni profiles (mainly in CIV 1549); ii) Weak P-Cygni features and iii) 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 1238 and/or C IV 1549. 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 WELS presenting P-Cygnis, most of the terminal velocities fall in the range ~1000-1500 km/s, while [WC]-PG 1159 stars possess much higher values, of about 3000 km/s. The [WC]-PG1159 stars are characterized by intense, simultaneous P-Cygni emissions in the 1150-2000A interval of N V 1238, O V 1371 and C IV 1549. In contrast, we found that O V 1371 is very weak or absent in the WELS spectra. On the basis of the ultraviolet spectra alone, our findings indicate that [WC]-PG 1159 stars are distinct from the WELS, contrary to previous claims in the literature.