FAR-ULTRAVIOLET OBSERVATIONS OF THE CRAB-NEBULA USING THE HOPKINS ULTRAVIOLET TELESCOPE

We have observed the far-ultraviolet spectrum (912-1860 angstrom) of the central portion of the Crab Nebula supernova remnant using the Hopkins Ultraviolet Telescope aboard the Astro-1 space shuttle mission in 1990 December. The synchrotron power-law continuum, greatly attenuated by interstellar absorption, can be seen extending down to approximately 1000 angstrom before being lost in the noise. The C IV lambda1549 emission line shows two resolved components representing the front and back sides of the shell expanding at roughly +/-1100 km s-1. This agrees well with the velocity distribution of optical [O III] emission at the same location in the Crab. The He II lambda1640 emission line, however, shows only a blueshifted component corresponding to the near side of the shell. These are the first observations to demonstrate that relative UV line intensity variations are present in the Crab Nebula. No other emission lines intrinsic to the Crab Nebula's filaments have been detected over the HUT wavelength range. We compare the HUT observation to optical emission-line imagery and to archival IUE observations. This comparison indicates that a distributed emission component, consisting of either a number of individually faint filaments or a diffuse component, produces the bulk of the emission seen by HUT. A grid of photoionization models for the Crab Nebula filaments is calculated to infer the range of ionization conditions and/or abundances that are needed to account for the observed UV line ratios and limits. Assuming that the diffuse redshifted and blueshifted emission seen with HUT arises in material of similar ionization parameter, the differing UV line ratios observed for these components probably indicates varying carbon and/or He abundances are present in the observed material.