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

TL;DR

This study derives empirical interstellar extinction curves from 5-20 micrometer Spitzer spectra toward star-forming regions, revealing changes in dust properties with increasing extinction, likely due to ice-mantled grain coagulation.

Contribution

It provides the first detailed empirical extinction curves in the 5-20 micrometer range for various extinction levels, linking curve shape changes to ice and grain coagulation processes.

Findings

Extinction curves resemble diffuse ISM for low extinction but differ at higher extinction levels.

Ice absorption features correlate with changes in the extinction curve shape.

Grain coagulation involving ice mantles likely causes the observed extinction curve variations.

Abstract

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.