The [ITAL]IUE[/ITAL] Spectrum of the “O [CSC]vi[/CSC] Sequence” Planetary Nebula He 2−55

Abstract

The first IUE observations of the "O VI Sequence" planetary nebula He 2-55 are reported. The ultraviolet spectrum is dominated by strong emission lines of O V, C IV, He II, and C III], plus a number of weaker ions for which emission line fluxes were measured. N V λ1240 and C IV λ1549 show P Cygni profiles. The stellar terminal wind velocity of 4500 ± 400 km s-1 is derived from the C IV λ1549 absorption. Interstellar extinction, E(B - V) = 0.54 ± 0.1, was determined from the observed flux of the He II λ1640/λ4686 lines. The only density diagnostic available from low-dispersion data, the Si III]/C III] emission-line ratio, yields an electron density of log Ne = 9.5 ± 0.5 cm-3 but usually results in higher log Ne than that derived from the C III] F(λ1907/λ1909) diagnostic that requires high-dispersion data.