Scattered Nebular Light in the Extended Orion Nebula

TL;DR

This study combines radio and optical data to analyze how scattered light affects the observed properties of the Extended Orion Nebula, revealing limitations in accurately determining nebular conditions and abundances.

Contribution

It demonstrates that scattered stellar light significantly contaminates nebular emission measurements, impacting the interpretation of physical conditions in the nebula and similar regions.

Findings

Scattered light increases with distance from Theta1OriC.

Wavelength-dependent scattering limits physical diagnostics.

Scattering dominates the eastern boundary of the nebula.

Abstract

We have combined 327.5 MHz radio observations and optical spectroscopy to study conditions in the Extended Orion Nebula. We see a steady progression of characteristics with increasing distance from the dominant photoionizing star Theta1OriC. This progression includes a decrease in the F(Halpha)/F(Hbeta) ratio, an increase in the relative strength of scattered stellar continuum, decrease in electron density determined from the [S II] doublet, and increase in the ratio of Emission Measures derived from the Hbeta line and the 327.5 MHz radio continuum. We conclude that beyond about 5' south of Theta1OriC that scattered light from the much brighter central Huygens region of the nebula significantly contaminates local emission. This strengthens earlier arguments that wavelength and model dependent scattering of emission line radiation imposes a fundamental limit on our ability to determine the physical conditions and abundances in this and arguably other similar Galactic Nebulae. The implications for the study of extra-galactic HII regions are even more severe. We confirm the result of an earlier study that at least the eastern boundary of the Extended Orion Nebula is dominated by scattered light from the Huygens region.