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 2MASS-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 \times 10 ^ { 21 } cm ^ { -2 } to 50 \times 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 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 ) , \sigma _ { e } ( 1200 ) , for every pixel .
From a value \sim 1 \times 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 \times 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 .