Large Scale Wind Driven Structures in the Orion Nebula

TL;DR

This study reveals the complex multi-shell structure of the Orion Nebula through spectroscopic imaging, identifying distinct expanding shells and their velocities, and clarifying the origins of observed emission features.

Contribution

It provides a detailed, multi-wavelength analysis of the Orion Nebula's structure, identifying multiple expanding shells and their dynamics, which was not previously understood.

Findings

Identification of the Outer Shell as part of the Veil-B HI layer

Discovery of the Inner Shell with a 27 km/s expansion velocity

Explanation of enhanced [C II] emission as radiation filtering effect

Abstract

A study of [C II] 158 micron and HI 21-cm spectroscopic images plus high velocity resolution optical and ultraviolet spectra has shown the structure of the Orion Nebula to be different from that found from the study of those data separately. The [C II] features recently identified as the [C II] Shell is shown to be part of the Veil-B HI foreground layer. Jointly called the Outer Shell, it covers the bright Huygens Region and the Extended Orion Nebula. Its maximum expansion velocity is 15 km/s. Closer to ThetaOne OrionisC there is a second expanding shell, called the Inner Shell. It has an expansion velocity of 27 km/s and probably results from a more recent period of strong wind from one or more of the Trapezium stars. Even closer to ThetaOne OrionisC there is a central high ionization bubble, freely expanding towards the observer but slowed in the opposite direction by photo-ionized gas coming off the Main Ionization Front. Utilization of spectroscopic measures of the equivalent width of Hbeta shows that the enhanced emission in [C II] seen just outside the visual wavelength boundaries of the Orion Nebula is not caused by limb-brightening of the Outer Shell. This enhanced emission is due to the radiation field of the Trapezium stars being filtered by intervening residual neutral hydrogen. A velocity component near 30 km/s (Heliocentric) first seen in HI is also present in [C II] and may result from a foreground cloud of the ISM.