Structure and Physical Conditions in the Huygens Region of the Orion Nebula

TL;DR

This study combines high-resolution imaging and spectroscopy to analyze the physical conditions and structure of the Orion Nebula's Huygens Region, revealing correlations between temperature, density, velocity, and turbulence, and their relation to the nebula's geometry.

Contribution

It provides the first detailed correlation of physical parameters in the main ionization front of the Orion Nebula using multi-instrument data, highlighting the impact of viewing angle and small-scale turbulence.

Findings

Radial velocity variations are mainly due to viewing angles of the ionization front.

Electron temperature and density increase closer to Theta1 Ori C.

Small-scale turbulence exists down to the resolution limit.

Abstract

HST images, MUSE maps of emission-lines, and an atlas of high velocity resolution emission-line spectra have been used to establish for the firrst time correlations of the electron temperature, electron density, radial velocity, turbulence, and orientation within the main ionization front of the nebula. From the study of the combined properties of multiple features, it is established that variations in the radial velocity are primarily caused by the photo-evaporating ionization front being viewed at different angles. There is a progressive increase of the electron temperature and density with decreasing distance from the dominant ionizing star Theta1 Ori C. The product of these characteristics (NexTe) is the most relevant parameter in modeling a blister-type nebula like the Huygens Region, where this quantity should vary with the surface brightness in Halpha. Several lines of evidence indicate that small-scale structure and turbulence exists down to the level of our resolution of a few arcseconds. Although photo-evaporative ow must contribute at some level to the well-known non-thermal broadening of the emission lines, comparison of quantitative predictions with the observed optical line widths indicate that it is not the major additive broad- ening component. Derivation of Te values for H+ from radio+optical and optical-only ionized hydro- gen emission showed that this temperature is close to that derived from [Nii] and that the transition from the well-known at extinction curve that applies in the Huygens Region to a more normal steep extinction curve occurs immediately outside of the Bright Bar feature of the nebula.