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

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We analyze the far-UV/UV spectra of four central stars of planetary nebulae (CSPNs) with strong wind features—NGC 2371, A78, IC 4776, and NGC 1535—and derive their photospheric and wind parameters by modeling high-resolution FUSE (Far Ultraviolet Spectroscopic Explorer) data in the far-UV and HST (Hubble Space Telescope) STIS and IUE data in the UV with spherical, non-LTE, line-blanketed model atmospheres. A78 is a hydrogen-deficient transitional [WR]-PG 1159 object, and we find NGC 2371 to be in the same stage, both migrating from the constant-luminosity phase to the white dwarf cooling sequence, with Teff 120 kK, 5 × 10-8 M☉ yr-1. NGC 1535 is a hydrogen-rich O(H) CSPN, and the exact nature of IC 4776 is ambiguous, although it appears to be helium-burning. Both objects lie on the constant-luminosity branch of post-AGB evolution and have Teff 65 kK, 1 × 10-8 M☉ yr-1. Thus, both the H-rich and H-deficient channels of PN evolution are represented in our sample. We also investigate the effects of including higher ionization stages of iron (up to Fe X) in the model atmosphere calculations of these hot objects (usually neglected in previous analyses) and find 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.