Spectrophotometry of the Huygens Region of the Orion Nebula, the Extended Orion Nebula, and M~43; Scattered Light Systematically Distorts Conditions Derived from Emission-Lines

Abstract

We report on medium resolution spectrophotometry of the Orion Nebula region, including for the first time the Extended Orion Nebula and the nearby M~43. The 49 long slit observations were divided into 99 smaller samples, which have allowed determinations of the amount of extinction, extinction corrected \Hbeta\ surface brightness, electron temperatures (from [S~II], [N~II], and [O~III]), and electron densities (from [S~II] and [Cl~III]) throughout much of this complex region. We verify an earlier conclusion from a radio/optical study that beyond about 5\arcmin\ from \ori\ local emission begins to be contaminated by scattering of light from the much brighter central Huygens Region of M~42 and this scattered light component becomes dominant at large distances. This contamination means that the derived properties for the outer regions are not accurate. From comparison of the light from the dominant star in M~43 with the continuum of that nebula (which is almost entirely scattered star light) it is determined that scattered light is enhanced in the blue, which can lead to observed Balmer line ratios that are theoretically impossible and erroneous electron temperatures. This blue scattering of emission-lines is important even in the Huygens Region because it means that at anything except very high spectroscopic resolution the observed lines are a blend of the original and scattered light, with shorter wavelength lines being artificially enhanced. This can lead to over-estimates of the electron temperatures derived from the nebular and auroral line ratios of forbidden lines. This phenomenon is probably applicable to many other H~II regions.

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