The wavelength-dependent mass absorption coefficient (opacity
abs) is a
critical parameter in the determination
of the total dust mass of IR-emitting dusty regions.
The dust opacity shows marked variation with local conditions.
Due to the incomplete understanding of the size, shape,
composition and structure of dust grains, our knowledge
of the long wavelength dust opacity is subject to large
uncertainties. We apply the Kramers-Kronig relation to
place a lower limit on the exponent index
and
an upper limit on the absolute value of the opacity
(Section 2), if the dust opacity is described as
a power-law function of wavelength
(abs ~
-).
Our current knowledge of the wavelength dependence exponent index
(Section 3) and the absolute values
of the opacity (Section 4) of interstellar dust
is summarized in the context of interstellar grain models,
laboratory measurements, and direct comparison of
the short-wavelength extinction with the long wavelength thermal emission.
ACKNOWLEDGMENTS. I am grateful to C.C. Popescu and R.J. Tuffs for inviting me to give an invited talk at this stimulating conference from which I learned much. I also thank E. Dwek, C.C. Popescu, R.J. Tuffs, A.N. Witt, and E.M. Xilouris for helpful discussions.