The Relationship Between The Optical Depth Of The 9.7 ?m Silicate Absorption Feature And Infrared Differential Extinction In Dense Clouds
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We have examined the relationship between the optical depth of the 9.7 mm silicate absorption feature (tau(9.7)) and the near-infrared color excess E(J - K-s), in the Serpens, Taurus, IC 5146, Chameleon I, Barnard 59, and Barnard 68 dense clouds/cores. Our data set, based largely on Spitzer IRS spectra, spans E(J - K-s) p = 0.3-10 mag (corresponding to visual extinction between about 2 and 60 mag). All lines of sight show the 9.7 mu m silicate feature. Unlike in the diffuse ISM where a tight linear correlation between the 9.7 mu m silicate feature optical depth and the extinction (A(V)) is observed, we find that the silicate feature in dense clouds does not show a monotonic increase with extinction. Thus, in dense clouds, tau(9.7) is not a good measure of total dust column density. With few exceptions, the measured values fall well below the diffuse ISM correlation line for E(J - K-s) > 2 mag (AV > 12 mag). Grain growth via coagulation is a likely cause of this effect.