THE PHYSICAL DIAGNOSTICS AND IONIZATION STRUCTURE OF THE ORION NEBULA

Abstract

ABSTRACT 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 ~5 arcminutes from the Trapezium and clearly show gradients in Te and Ne. Temperatures are derived using S+, O+, N+, O++ and the Balmer Discontinuity, TBac. Values for T(S+) 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 TBac is derived with temperatures as low as 3000 K and is left open to interpretation. Density gradients for S+, O+ and Cl++ are also presented and are consistent with model predictions for an H II 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, t2, for the ions O++ and N+ 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 TBac predicts t2 values of 0.01-0.15 with values increasing outward. The ion S+ gives unusually high values of t2 and is not a reliable indicator of fluctuations in the H+ zone since some of the S II emission arises from beyond the ionization front.