An Optical Study of the Circumstellar Environment Around the Crab Nebula

Abstract

Long-slit spectra of two peripheral regions around the Crab Nebula showno H{α} emission down to a flux level of 1.5 x 10(-7) erg cm(-2)s(-1) sr(-1) (0.63 Rayleigh), corresponding to an emission measure limitof 4.2 cm(-6) pc (3sigma ) assuming A_V = 1.6(m) and T_e = 7000 K. Thisis below the flux levels reported by Murdin {\amp} Clark [Nature, 294,543 (1981)] for a H{α} halo around the Crab. Narrow H{β} emissionas described by Murdin [MNRAS, 269, 89 (1994)] is detected but appearsto be galactic emission unassociated with the remnant. A review of priorsearches indicates no convincing observational evidence to supporteither a high- or low-velocity envelope around the remnant. Spectralscans confirm a well-organized, N-S expansion asymmetry of the filamentswith a ~ 500 km s(-1) central velocity constriction as described byMacAlpine et al. [ApJ, 342, 364 (1989)] and Lawrence et al. [AJ, 109,2635 (1995)] but questioned by Hester et al. [ApJ, 448, 240 (1995)]. Thevelocity pinching appears to coincide with an east-west chain of bright[O III] and helium-rich filaments. This expansion asymmetry might be theresult of ejecta interaction with a disk of circumstellar matter, butsuch a model may be inconsistent with H and He filament abundances inthe velocity constriction zone. A re-analysis of the remnant's totalmass suggests that the filaments contain 4.6 +/- 1.8 M_{sun} inionized and neutral gas, about twice that of earlier estimates. For a 10M_{sun} progenitor, this suggests that =~ 4 M_{sun}remains to be detected in an extended halo or wind.