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.
