Fluorescence of [Fe II] in H II regions

TL;DR

This paper investigates the fluorescence of [Fe II] lines in H II regions M42 and M43, demonstrating that their intensities are correlated with nebular continuum, providing evidence for fluorescent excitation in line formation.

Contribution

It provides the first direct evidence linking [Fe II] line intensities to fluorescent excitation caused by scattered stellar radiation in H II regions.

Findings

[Fe II] line intensities correlate with nebular continuum.

Fluorescent excitation is confirmed as a key process.

Evidence is independent of physical conditions.

Abstract

A study of [Fe II] lines at various positions within the H II regions M42 and M43 is presented. The relative intensities of selected optical [Fe II] lines are shown to be correlated with the intensity of the apparent nebular continuous spectrum. Since the continuum of H II regions is known to be mostly stellar radiation scattered by dust intermixed with the emitting gas, these correlations provide direct evidence for the existence of fluorescent excitation in the formation process of the [Fe II] lines, irrespective of the prevailing physical state.