Discovery of Ne VII in the winds of hot evolved stars

We show that a strong P Cygni feature seen in the far-UV spectra of some very hot (T-eff >= 85 kK) central stars of planetary nebulae (CSPN), which has been previously identified as C (III) lambda 977, actually originates from Ne (VII) lambda 973. Using stellar atmosphere models, we reproduce this feature seen in the spectra of two [WR] PG 1159-type CSPN, A78 and NGC 2371, and in one PG 1159 CSPN, K1-16 ( the [ WR]'' nomenclature denotes a CSPN with spectral features similar to those of Wolf-Rayet stars). In the latter case, our analysis suggests an enhanced neon abundance. Strong neon features in CSPN spectra are important because an overabundance of this element is indicative of processed material that has been dredged up to the surface from the intershell region in the born-again'' scenario, an explanation of hydrogen-deficient CSPN. Our modeling indicates that the Ne (VII) lambda 973 wind feature may be used to discern enhanced neon abundances for stars showing an unsaturated P Cygni profile, such as some PG 1159 stars. We explore the potential of this strong feature as a wind diagnostic in stellar atmosphere analyses for evolved objects. For the [ WR] PG 1159 objects, the line is present as a P Cygni line for T-eff >= 85 kK and becomes strong for 100 <= T-eff <= 155 kK when the neon abundance is solar, and can be significantly strong beyond this range for higher neon abundances. When unsaturated, i. e., for very high Teff and/or very low mass-loss rates, it is sensitive to. M and very sensitive to the neon abundance. The Ne (VII) classification is consistent with recent identification of this line, seen in absorption in many PG 1159 spectra.