Using Spitzer Infrared Spectrograph observations of G0–M4 III stars behind dark clouds , I construct 5 - 20 \mu m empirical extinction curves for 0.3 \leq A _ { K } < 7 , which is equivalent to A _ { V } between \approx 3 and 50 .
For A _ { K } < 1 the curve appears similar to the Mathis ( 17 ) diffuse interstellar medium extinction curve , but with a greater degree of extinction .
For A _ { K } > 1 , the cuve exhibits lower contrast between the silicate and absorption continuum , developes ice absorption , and lies closer to the Weingartner & Draine ( 20 ) R _ { V } = 5.5 case B curve , a result which is consistent with that of Flaherty et al .
( 13 ) and Chiar et al .
( 8 ) .
Recently work using Spitzer Infrared Array Camera data by Chapman et al .
( 7 ) 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 9.7 \mu m silicate and 6.0 , 6.8 , and 15.2 \mu m 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 coagulation of ice-mantled grains rather than ice-mantled grains alone .