Changes of Dust Opacity with Density in the Orion A Molecular Cloud

We have studied the opacity of dust grains at submillimeter wavelengths by estimating the optical depth from imaging at 160, 250, 350, and 500 μm from the Herschel Gould Belt Survey and comparing this to a column density obtained from the Two Micron All Sky Survey derived color excess E(J - K _s). Our main goal was to investigate the spatial variations of the opacity due to "big" grains over a variety of environmental conditions and thereby quantify how emission properties of the dust change with column (and volume) density. The central and southern areas of the Orion A molecular cloud examined here, with N _H ranging from 1.5 × 10²¹ cm^-2 to 50 × 10²¹ cm^-2, are well suited to this approach. We fit the multi-frequency Herschel spectral energy distributions (SEDs) of each pixel with a modified blackbody to obtain the temperature, T, and optical depth, τ₁₂₀₀, at a fiducial frequency of 1200 GHz (250 μm). Using a calibration of N _H/E(J - K_s ) for the interstellar medium (ISM) we obtained the opacity (dust emission cross-section per H nucleon), σ_e(1200), for every pixel. From a value ~1 × 10^-25 cm² H^-1 at the lowest column densities that is typical of the high-latitude diffuse ISM, σ_e(1200) increases as N ^0.28 _H over the range studied. This is suggestive of grain evolution. Integrating the SEDs over frequency, we also calculated the specific power P (emission power per H) for the big grains. In low column density regions where dust clouds are optically thin to the interstellar radiation field (ISRF), P is typically 3.7 × 10^-31 W H^-1, again close to that in the high-latitude diffuse ISM. However, we find evidence for a decrease of P in high column density regions, which would be a natural outcome of attenuation of the ISRF that heats the grains, and for localized increases for dust illuminated by nearby stars or embedded protostars.