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 um from the
Herschel Gould Belt Survey and comparing this to a column density obtained from
the 2MASS-derived color excess E(J-Ks). 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 NH ranging from 1.5X10^21 cm^-2
to 50X10^21 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 um). Using a calibration of NH/E(J-Ks)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 of
~ 1X10^-25 cm^2 H^-1 at the lowest column densities that is typical of the high
latitude diffuse ISM, \sigma_e(1200) increases as NH^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.
