Browsing by Author "Walter, Donald Kim"

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    The dust distribution and ionization structure of the Orion nebula
    (1991) Walter, Donald Kim; Dufour, Reginald J.
    Presented are the results of an empirical study of the scattering of starlight in the UV and optical continuum by dust in Orion, plus the distribution of the ionic species of CNO across the face of the nebula. Spectra from the IUE have been combined with ground-based spectrophotometry and CCD imagery. This study of M42 is the most extensive to date. The distribution of dust and its effects on the scattering of starlight are examined. The variation of scattered light with distance and wavelength has been parameterized and a functional relationship determined. Abundances for ionic species of carbon, oxygen and nitrogen have been calculated. Several results were uncovered. The C/H abundance in Orion must be lowered by a factor of 3-5 compared to past results. The C/H abundance is constant across the inner 5 arcmin of the nebula as is the C/O ratio.
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    The physical diagnostics and ionization structure of the Orion Nebula
    (1993) Walter, Donald Kim; Dufour, Reginald J.
    The most detailed study to date of the physical diagnostics and ionization structure of the core of M42 (the Orion Nebula) is presented using new long slit spectra and existing CCD imagery. Cospatial temperature and density diagnostics are presented for distances out to $\sim$5 arcminutes from the Trapezium and clearly show gradients in $T\sb{e}$ and $N\sb{e}.$ Temperatures are derived using S$\sp+,$ O$\sp+,$ N$\sp+,$ O$\sp{++}$ and the Balmer Discontinuity, T(Bac). Values for T(S$\sp+)$ in the range 11,000-14,500 K are presented and suggest shock heating at the ionization front as predicted by some models. A gradient in T(Bac) is derived with temperatures as low as 3000 K and is left open to interpretation. Density gradients for S$\sp+,$ O$\sp+$ and Cl$\sp{++}$ are also presented and are consistent with model predictions for an H scII region in the champagne phase of evolution. Using imagery we have constructed a picture of the core of Orion, including previously unknown features. Ratio maps show a high ionization cavity centered on the Trapezium and a low ionization region to the southwest. Ripples and bends in the background molecular cloud are detected. Further to the southwest the ionization begins to increase with distance, which is interpreted as the molecular cloud curving back toward the observer. Additional features in the imagery include a bar-like structure to the northeast of the Trapezium and a high density knot and ellipse of material believed to be a Herbig Haro object. Our diagnostics provide unprecedented detail as to the variation of temperature fluctuations with position. The mean square temperature fluctuation, $t\sp2,$ for the ions O$\sp{++}$ and N$\sp+$ is similar on a global scale, with a value in the range 0.03-0.05 for an assumed constant recombination temperature. Using a gradient in T(Bac) predicts $t\sp2$ values of 0.01-0.15 with values increasing outward. The ion S$\sp+$ gives unusually high values of $t\sp2$ and is not a reliable indicator of fluctuations in the H$\sp+$ zone since some of the (S II) emission arises from beyond the ionization front.