OBSERVATIONAL 5-20 μm INTERSTELLAR EXTINCTION CURVES TOWARD STAR-FORMING REGIONS DERIVED FROM <i>SPITZER</i> IRS SPECTRA

Abstract

Using Spitzer Infrared Spectrograph observations of G0–M4 III stars behind dark clouds, I construct 5 − 20µm empirical extinction curves for 0.3 ≤ AK &amp;lt; 7, which is equivalent to AV between ≈ 3 and 50. For AK &amp;lt; 1 the curve appears similar to the Mathis (1990) diffuse interstellar medium extinction curve, but with a greater degree of extinction. For AK&amp;gt; 1, the cuve exhibits lower contrast between the silicate and absorption continuum, developes ice absorption, and lies closer to the Weingartner &amp;amp; Draine (2001) RV = 5.5 case B curve, a result which is consistent with that of Flaherty et al. (2007) and Chiar et al. (2007). Recently work using Spitzer Infrared Array Camera data by Chapman et al. (2009) independently reaches a similar conclusion, that the shape of the extinction curve changes as a function of increasing AK. By calculating the optical depths of the 9.7µm silicate and 6.0, 6.8, and 15.2 µm 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. Subject headings: infrared: stars−ISM: clouds−stars: formation