Motions and Structure of the Filamentary Envelope of the Crab Nebula

Abstract

Proper motions have been measured for 132 line-emitting filaments in the Crab Nebula on direct plates taken with the 100- and 200-in. telescopes. These motions, if assumed constant and extrapolated backward in time, converge toward a point about 12" southeast of the double star near the center of the nebula. The filaments approach, on the average, most closely to that point or expansion center in about the year 1140, indicating that the expansion has been somewhat accelerated. Proper motions and radial velocities were measured for an additional 126 features on the same direct plates and Mt. Wilson and Palomar spectra. These, along with other, nondynamical data, indicate a most probable distance to the object of 2.02 kpc. Two projections of the nebula perpendicular to the one seen in the plane of the sky are constructed and do not differ significantly from that presented to us. The proper motion and radial velocity of the nebula as a whole are found and converted into galactic coordinates. This requires several assumptions of questionable validity. The nebula seems to be moving about 114 km/sec faster than the galactic rotation at its position. The proper motion of the so-called central star translates into an equally unlikely space motion, but this is also extremely uncertain. The likelihood of a physical connection between the star and the nebula is discussed. The line-emitting filaments are shown to be distributed throughout the nebula rather than being confined to a thin outer envelope. The motions are largely radial, each filament having a velocity approximately proportional to its distance from the expansion center. The deviations from this proportionality are as large as 0.032"/yr or 300 km/sec and have mean values near 0.010"/yr or 70 km/sec. These deviations are correlated with the scatter of the filaments around the expansion center in 1140.

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