The Crab Nebula: a Theoretical and Observational Study.

The Crab Nebula is the remnant of a supernova event which occurred in 1054 AD. Because of its relatively young age, the filamentary gas contained in the Crab has not become substantially diluted by interstellar material. Thus, a detailed chemical abundance study should yield some information about the make-up of the stellar precursor. This thesis is a study of the Crab Nebula filaments. It includes an analysis of published optical spectrophotometric observations using detailed photoionization models plus the presentation of additional near infrared and near ultraviolet data. The model analysis employed a previously-existing photoionization code to calculate emission line strengths as a function of various gas parameters. The near infrared observations were obtained with the CCD camera and the 4-M telescope at Kitt Peak National Observatory (KPNO) between (lamda)(lamda)6500-10200 in an attempt to measure the strengths of emission lines which the model calculations indicated were important. The near ultraviolet observations covered the spectral range (lamda)(lamda)3200-5000 and were made using the IIDS and the 2.1-M telescope at KPNO. The last set of data were important for establishing the validity of the on-the-spot assumption made in the calculations with respect to the fate of He II (lamda)304, by trying to measure O III (lamda)3444, produced by the Bowen fluorescence mechanism. The major findings of this study were: (1) Filaments with strong He I (lamda)5876 emission have a relative helium abundance by mass of roughly 60% or more; (2) The relative helium abundance may vary among the filaments; (3) The strong {S II} (lamda)(lamda)6716,6731 emission is due to the presence of both a large amount of neutral gas and a power law ionizing spectrum; (4) The relative heavy element abundances are roughly solar, although O and N may be several times underabundant while Ni may be overabundant by an order of magnitude; (5) {S III} (lamda)9532/{S II} (lamda)6731 is observed to vary directly with {O III} (lamda)5007/{O II} (lamda)3727, while the models suggest an inverse correlation; (6) Strong {C I} (lamda)(lamda)9823,9850 emission may be due to collisional excitation by H(DEGREES); (7) The Bowen mechanism efficiency is less than or equal to that observed in planetary nebulae, suggesting that He II (lamda)304 does not enter the O('+2) zone but is probably reflected at the He('+2)(--->)He('+) interface.Ph.D.AstronomyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/159390/1/8314290.pd