Turbulence in the Ionized Gas of the Orion Nebula

TL;DR

This study investigates turbulence in the Orion Nebula's ionized gas using high-resolution spectroscopy and statistical analysis, revealing a Kolmogorov-like turbulence spectrum driven by dense core autocorrelation scales.

Contribution

It identifies the dominant driving scale of turbulence as the autocorrelation length of dense cores, and quantifies the contributions of turbulence and champagne flows to velocity dispersion.

Findings

Velocity power spectrum aligns with Kolmogorov theory between 8 and 22 arcsec scales.

Turbulence is subsonic, contributing about half of the ionized density variance.

Ionized gas is confined to a thick shell, not filling the nebula's interior.

Abstract

In order to study the nature, origin, and impact of turbulent velocity fluctuations in the ionized gas of the Orion Nebula, we apply a variety of statistical techniques to observed velocity cubes. The cubes are derived from high resolving power ($R \approx 40,000$) longslit spectroscopy of optical emission lines that span a range of ionizations. From Velocity Channel Analysis (VCA), we find that the slope of the velocity power spectrum is consistent with predictions of Kolmogorov theory between scales of 8 and 22 arcsec (0.02 to 0.05 pc). The outer scale, which is the dominant scale of density fluctuations in the nebula, approximately coincides with the autocorrelation length of the velocity fluctuations that we determine from the second order velocity structure function. We propose that this is the principal driving scale of the turbulence, which originates in the autocorrelation length of dense cores in the Orion molecular filament. By combining analysis of the non-thermal line widths with the systematic trends of velocity centroid versus ionization, we find that the global champagne flow and smaller scale turbulence each contribute in equal measure to the total velocity dispersion, with respective root-mean-square widths of 4-5 km/s. The turbulence is subsonic and can account for only one half of the derived variance in ionized density, with the remaining variance provided by density gradients in photoevaporation flows from globules and filaments. Intercomparison with results from simulations implies that the ionized gas is confined to a thick shell and does not fill the interior of the nebula.