THE IUE SPECTRUM OF THE O-VI-SEQUENCE PLANETARY-NEBULA HE-2-55

The first IUE observations of the ''O VI Sequence'' planetary nebula H e 2-55 are reported. The ultraviolet spectrum is dominated by strong e mission 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 lambda 1240 an d C IV lambda 1549 show P Cygni profiles. The stellar terminal wind ve locity of 4500 +/- 400 km s(-1) is derived from the C IV lambda 1549 a bsorption. Interstellar extinction, E(B - V) = 0.54 +/- 0.1, was deter mined from the observed flux of the He II lambda 1640/lambda 4686 line s. The only density diagnostic available from low-dispersion data, the Si III]/C III] emission-line ratio, yields an electron density of log N-e = 9.5 +/- 0.5 cm(-3) but usually results in higher log N-e than t hat derived from the C III] F(lambda 1907/lambda 1909) diagnostic that requires high-dispersion data.