The Crab Nebula. I. Spectrophotometry of the filaments.

Relative line intensities in the 3700-7400 A region are obtained from new spectrophotometric measurements of ten positions in the Crab Nebula. O III forbidden line intensities imply electron temperatures of 11,000-18,300 K, while generally lower temperatures are indicated by the O II, N II and S II forbidden lines. The 6717 and 6731 A forbidden S II line ratio yields electron density estimates of 550-3500/cu cm, with a typical filament value of 1300 cu cm. While Balmer decrements are generally consistent with pure recombination, some filaments show steeper decrements. He line intensities are found to have significant variation. The presence of a moderately strong line at 7378 A, which may be due to the Ni II forbidden line is confirmed. Some filaments are found to exhibit a wide range of ionization states.