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Description
Using Spitzer Infrared Spectrograph observations of G0-M4III stars behind dark clouds, I construct 5-20um empirical extinction curves for 0.3<=A_K_<7, which is equivalent to A_V_ between ~3 and 50. For A_K_<1, the curve appears similar to the Mathis diffuse interstellar medium extinction curve, but with a greater degree of extinction. For A_K_>1, the curve exhibits lower contrast between the silicate and absorption continuum, develops ice absorption, and lies closer to the Weingartner and Draine R_V_=5.5 Case B curve, a result which is consistent with that of Flaherty et al. (2007ApJ...663.1069F) and Chiar et al. (2007ApJ...666L..73C). Recently, work using Spitzer Infrared Array Camera data by Chapman et al. independently reaches a similar conclusion that the shape of the extinction curve changes as a function of increasing A_K_. By calculating the optical depths of the 9.7um silicate and 6.0, 6.8, and 15.2um ice features, I determine that a process involving ice is responsible for the changing shape of the extinction curve and speculate that this process is a coagulation of ice-mantled grains rather than ice-mantled grains alone.
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