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Session 18 - The Interstellar Medium.
Oral session, Monday, June 08

[18.05] Emission from Shocked Gas and Photoionized Gas in the Crab Nebula

R. Sankrit (Arizona State University)

The [O III] skin around the Crab Nebula traces the Rayleigh-Taylor unstable interface between the synchrotron nebula and swept-up ejecta from the supernova remnant explosion (Hester et al. 1996, ApJ, 456, 225). We show that the skin is most easily understood as the cooling region behind a shock driven into an extended remnant of freely expanding ejecta by the pressure of the Crab synchrotron nebula. Shock models, constrained by observed properties of the Crab, correctly predict the [O III] \lambda5007 brightness of the skin. The models also predict the strength of C IV \lambda1549 emission from the skin and the remarkably strong [Ne V] \lambda3426 at two locations reported in the literature. While it is possible to match the [O III] brightness with ad hoc photoionization models, such models generally underpredict C IV and [Ne V] by an order of magnitude or more (Sankrit amp; Hester 1997, ApJ, 491, 796).

We present narrow band images of the Crab Nebula taken with the Hubble Space Telescope-WFPC2 which show the morphology and ionization structure of the photoionized filaments in great detail. Low ionization emission from the filaments is found to be concentrated in very sharp structures while high ionization emission is much more diffuse. Individual filaments are found to lie along a sequence of ionization structure ranging from features in which all lines are concentrated in the same compact volume through features with low ionization cores surrounded by high ionization envelopes. Photoionization models of cylindrically symmetric filaments with varying density profiles match the observed variation in filament ionization and emission properties. A photoionization model of a uniform, low density medium matches the extended diffuse component which dominates the high ionization emission (Sankrit et al. 1998, ApJ, in press). We find that ground-based spectra of the Crab generally integrate over a number of discrete small scale structures. This places a fundamental limit on what can be inferred reliably from comparison of spectra with 1-D photoionization models.

Program listing for Monday