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Analysis of Far-UV Data of Central Stars of Planetary Nebulae: Occurrence and Variability of Stellar Winds

The occurrence of stellar wind in central stars planetary nebulae (CSPNe) can be revealed by the presence of P Cygni profiles of high-excitation lines overimposed on the stellar continuum. We have examined the entire FUSE archive and merged all useful observations of CSPNe to produce the highest quality spectra that can be used to assess the occurrence of stellar winds. Furthermore, the individual spectra of each CSPN have been searched for variability in the P Cygni profile. P Cygni profiles of high-excitation lines have been found in 44 CSPNe, with a clear correlation between the line ionization potential and the star effective temperature. We introduce a prescription to derive the terminal wind velocity (v_inf) from saturated and unsaturated P Cygni profiles and provide new values of v_inf for these stars. Another 23 CSPNe do not show P Cygni profiles or their FUSE data are not conclusive to determine the occurrence of P Cygni profiles. Variability in the P Cygni profile of high-excitation far-UV lines is found for the first time in six CSPNe, namely Hen 2-131, NGC 40, NGC 1535, NGC 2392, Sp 3, and SwSt 1, increasing up to 13 the number of CSPNe with variable P Cygni profiles in the UV, including those previously reported using IUE or FUSE observations. Variability is seen preferentially in the unsaturated P V and Si IV lines, but also in saturated C III and O VI lines. The CSPNe with variable P Cygni profiles have similar stellar properties (relatively low log(g) and T_eff) that suggests they are less evolved CSPNe. Some of the CSPNe with variable P Cygni profiles show O VI lines, while their T_eff is insufficient to produce this ion. We suggest that such ion is produced by Auger ionization from X-rays associated to shocks in their stellar winds as is the case in massive OB stars of high ionization potential ions which cannot be abundantly produced by photoionizations.