The Nature of [Ar III]-Bright Knots in the Crab Nebula

Abstract

The kinematic and morphological properties of a string of [Ar III]-bright knots in the Crab Nebula are examined using 1994-1999 Hubble Space Telescope (HST) Wide Field Planetary Camera 2 images of the remnant. We find that five southern [Ar III]-bright knots exhibit ordinary radial motions away from the nebula's center of expansion with magnitudes consistent with their projected radial displacements. This result does not support the suggestion by MacAlpine et al. (published in 1994) that these knots might be moving rapidly away from the Crab pulsar because of a collimated wind. The HST images also do not show that the [Ar III] knots have unusual morphologies relative to other features in the remnant. Our proper-motion results, when combined with radial velocity estimates, suggest these knots have relatively small space velocities, implying locations relatively close to the central source of ionizing radiation. This might lead to higher knot gas temperatures and thereby explain the knots' unusual line emission strengths as MacAlpine et al. suspected.