Observational $5-20μ$m Interstellar Extinction Curves Toward Star-Forming Regions Derived from Spitzer IRS Spectra

Details Observational $5-20μ$m Interstellar Extinction Curves Toward Star-Forming Regions Derived from Spitzer IRS Spectra Observational $5-20μ$m Interstellar Extinction Curves Toward Star-Forming Regions Derived from Spitzer IRS Spectra

2008-10-24

arxiv.org/abs/0810.4561v2

Astrophys.J.693:L81-L85,2009

Astronomy and Astrophysics

Astrophysics

Submitted to ApJ Letters, 5 pages, 4 figures, 1 table. Revised version published in ApJ Letters, with 5 pages, 3 figures, and

Scientific paper

10.1088/0004-637X/693/2/L81

Using \emph{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 \citet{mathis90} 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, developes ice absorption, and lies closer to the \citet{wd01} $R_V=5.5$ case B curve, a result which is consistent with that of \citet{flaherty07} and \citet{chiar07}. Recently work using \emph{Spitzer} Infrared Array Camera data by \citet{chapman08} 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.

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