Physics
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt.........4s&link_type=abstract
Thesis (PHD). ARIZONA STATE UNIVERSITY , Source DAI-B 59/06, p. 2804, Dec 1998, 156 pages.
Physics
Crab Nebula Supernova, Eagle Nebula, Hubble Space Telescope
Scientific paper
Narrow band images of the Crab Nebula supernova remnant and of the Eagle Nebula H II region taken with the Hubble Space Telescope (HST) show the ionization structure of the emitting gas in unprecedented detail because of the high spatial resolution. The physics of the emission processes-shock excited emission and photoionized emission-is well understood. Sophisticated numerical codes are used to model the ionization structure and emission observed in these images. It is found that the thin skin of material around the Crab synchrotron nebula visible in (O III) λ5007 emission is best explained as the cooling region behind a shock driven by the synchrotron nebula into a surrounding remnant of freely expanding ejecta. Shock models, with parameters derived from independently known properties of the Crab, explain the observed spectrum of the skin while photoionization models fail to explain the observed strength of high ionization lines such as C IV λ1549. This result is clear evidence that the synchrotron nebula is interacting with an extended remnant of ejecta, which in turn has significant implications for the structure and evolution of the Crab. At HST resolution, it is seen that low ionization emission, from lines such as (O I) λ6300, is concentrated in sharp structures while high ionization emission (from (O III) λ5007) 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 symmetrical filaments with varying 'core-halo' density profiles can match the observed variation in the filament structure in the Crab. A photoionization model of a uniform low density medium matches the extended diffuse component which dominates the high ionization emission. It is found that detailed knowledge of the filament structures present in an aperture is needed to correctly interpret ground-based spectra of the Crab. The images also show that many filament cores coincide with dust extinction features, which suggest that the dust to gas mass ratio may be up to an order of magnitude higher than is typical in the interstellar medium. Nebula show the interface between the ionized gas and the molecular cloud in tangency against the background of the ionized cavity which constitutes the H II region. A photoionization model using a density profile for the photoevaporative flow that is expected at such an interface is successful at explaining the observed emission profiles of Hα λ6563, (S II) λλ6716,6731, and (O III) λ5007. The ionizing flux is well constrained by the Hα emission and the sulphur abundance is constrained by the peak of the (S II) emission. A grid of models using the same density profiles shows how various emission properties depend on the ionizing continuum shape, ionizing flux and elemental abundances.
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