The Internal Nebular Attenuation Curve of Three-Dimensional Turbulent HII regions

TL;DR

This study models the internal dust attenuation in turbulent HII regions using Monte Carlo radiative transfer, revealing how turbulence and clumpy structures influence emission-line fluxes and attenuation curves.

Contribution

It introduces a new simulation approach with the M3D code to analyze the effects of turbulence and clumpiness on internal nebular attenuation curves.

Findings

The Hα/Hβ ratio is approximately 3.02-3.03, higher than the standard 2.86 due to cooler temperatures.

Clumpy structures do not alter the slope of the internal attenuation curve.

Turbulence affects line-of-sight obscuration and emission-line fluxes.

Abstract

The internal dust attenuation of the Hii region reduces the observed emission-line fluxes. Turbulent density fields within each Hii region change the degree of the line-of-the-sight obscuration of the emission-line fluxes. In this paper, we implement the dust Monte-Carlo radiative transfer in the latest M3D code, creating the emission-line maps attenuated by the internal turbulent dust obscuration with the varying Mach numbers. The internal density and temperature fluctuations of Hii regions make the radiative transfer of hydrogen lines neither Case A nor Case B conditions, resulting in the global H{\alpha} to H\b{eta} ratio of approximately 3.02-3.03, differing from the widely-used value of 2.86. This deviation from Case B is because the temperature of these Hii regions is cooler than 10,000 K. We further derive the internal nebular attenuation curve from the attenuated Hydrogen lines, finding that the clumpy structures within Hii regions do not change the slope of the internal attenuation curve. This is because the heavy dust obscuration of dense clumps is canceled out by the high in-situ production of emission-line intensities.