Physics
Scientific paper
Feb 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997taea....1.1001w&link_type=abstract
NASA University Research Centers Technical Advances in Education, Aeronautics, Space, Autonomy, Earth and Environment, vol. 1, p
Physics
Temperature Gradients, Electron Density (Concentration), Nebulae, Galactic Structure, Spatial Distribution, Spectrophotometry, Orion Nebula, Abundance, Charge Coupled Devices, Diagnosis, Electron Energy, Imagery, Lagoons, Sensitivity, Stars
Scientific paper
The results from spectrophotometric studies of several galactic emission nebulae are discussed. Details of the spatial variation of the electron temperature (Te) and density (Ne) are given for the Orion Nebula (M42), with a less in-depth discussion of the Lagoon Nebula (M8) and the Ring Nebula (M57), both of which are covered elsewhere in these proceedings by Anthony Williams and Latongia Groce respectively. The data set consists of ground based longslit spectra and CCD imagery taken through narrow-band interference filters centered on important diagnostic lines of O(sup ++), N(sup +) and S(sup +) and H(sup +). These seeing-limited (less than 2") images show significant spatial variations in Te and Ne on scales as small as 0.005 parsec. Global trends and local variations of these diagnostics will be examined. For the Orion Nebula it has been known for some time that Ne determined from the S+ double at lambda(lambda)6717,31 decreases with distance from the stellar ionizing source, theta(sup 1)C Ori. Our data show a similar trend for the Cl(sup -+) ion using the doublet at lambda(lambda)5518,5538. These results are consistent with models of H h regions in the champagne phase of evolution. The existence of temperature gradients in the Orion Nebula has been debated for over twenty years. We have used one, self-consistent set of data to convincingly show that these gradients do exist and are different for each ion. The O(sup ++) and S(sup +) data can be fit with a gradients which increase with distance from (theta)'C Ori, but do show local deviations from the fits. The value for Te of the N(sup +) ion shows a sharp drop in temperature out to a distance of 100 arcseconds from (theta)'C Ori and then a very gradual decrease to a low of 8500 K at a distance of 300 arcseconds. These results must be taken into account by future models of H h regions. Since nebular abundances are normally determined from forbidden-line ratios, and these lines are very sensitive to temperature (proportional to exp [- 1/kT] ) a difference of only 500-1000 K can result in abundance calculations which differ by a factor of two or more. For nebular abundance calculations, the importance of using local, on-the-spot values for Te and Ne rather than global values is demonstrated.
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