The internal nebular attenuation curve of three-dimensional turbulent HII regions

The internal dust attenuation of the H II region reduces the observed emission-line fluxes. Turbulent density fields within each H II 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. Due to the internal density and temperature fluctuations of H II regions the radiative transfer of hydrogen lines meet neither Case A nor Case B conditions, resulting in the global H α to H β 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 H II 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 H II regions do not change the slope of the internal attenuation curve. This is because the beavy dust obscuration of dense clumps is canceled out by the high in situ production of emission-line intensities.