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 mu m 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 x 10(21) cm(-2) to 50 x 10(21) 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, tau(1200), at a fiducial frequency of 1200 GHz (250 mu m). 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), sigma(e)(1200), for every pixel. From a value similar to 1 x 10(-25) cm(2) H-1 at the lowest column densities that is typical of the high-latitude diffuse ISM, sigma(e)(1200) increases as N-H(0.28) 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 x 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.