The Relationship between the Optical Depth of the 9.7 μm Silicate Absorption Feature and Infrared Differential Extinction in Dense Clouds

We have examined the relationship between the optical depth of the 9.7 μm silicate absorption feature (τ 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 ) = 0.3-10 mag (corresponding to visual extinction between about 2 and 60 mag). All lines of sight show the 9.7 μm silicate feature. Unlike in the diffuse ISM where a tight linear correlation between the 9.7 μm 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, τ 9.7 is not a good measure of total dust column density. With few exceptions, the measured τ 9.7 values fall well below the diffuse ISM correlation line for E ( J - K s ) > 2 mag ( A V > 12 mag). Grain growth via coagulation is a likely cause of this effect.