Far‐Ultraviolet Spectroscopic Analyses of Four Central Stars of Planetary Nebulae

We analyze the Far-UV/UV spectra of four central stars of planetary nebulaewith strong wind features -- NGC 2371, Abell 78, IC 4776 and NGC 1535, andderive their photospheric and wind parameters by modeling high-resolution FUSE(Far-Ultraviolet Spectroscopic Explorer) data in the Far-UV and HST-STIS andIUE data in the UV with spherical non-LTE line-blanketed model atmospheres.Abell 78 is a hydrogen-deficient transitional [WR]-PG 1159 object, and we findNGC 2371 to be in the same stage, both migrating from the constant-luminosityphase to the white dwarf cooling sequence with Teff ~= 120 kK, Mdot ~= 5x10^-8Msun/yr. NGC 1535 is a ``hydrogen-rich'' O(H) CSPN, and the exact nature of IC4776 is ambiguous, although it appears to be helium burning. Both objects lieon the constant-luminosity branch of post-AGB evolution and have Teff ~= 65 kK,Mdot ~= 1x10^-8 Msun/yr. Thus, both the H-rich and H-deficient channels of PNevolution are represented in our sample. We also investigate the effects ofincluding higher ionization stages of iron (up to FeX) in the model atmospherecalculations of these hot objects (usually neglected in previous analyses), andfind iron to be a useful diagnostic of the stellar parameters in some cases.The Far-UV spectra of all four objects show evidence of hot (T ~ 300 K)molecular hydrogen in their circumstellar environments.Comment: 38 pages, 8 figures (6 color). Accepted for publication in Ap