Annu. Rev. Astron. Astrophys. 1990. 28: 37-70 Copyright © 1990 by Annual Reviews. All rights reserved

9. SUMMARY

``Interstellar dust'' refers to materials with rather different properties, and the ``mean extinction law'' of Seaton (149) or Savage and Mathis (144) should be replaced by the expression given in CCM89, using the appropriate value of total-to-selective extinction, R_V. The older laws were appropriate for the diffuse ISM, but dust in clouds differs dramatically in its extinction law (Figure 2). However, there are certainly real deviations from the mean extinction law (see error bars in the inset in Figure 2). The extinction law for > 0.9 µm seems to be independent of environment, to within the present observational errors. Other diagnostics of dust, especially the depletions from the gas phase, confirm that properties of the grains vary along various lines of sight, but only one parameter, probably related to the local gas density, determines the grain properties surprisingly well.

Dust is heavily processed while in the ISM by being included within clouds and cycled back into the diffuse ISM many times during their lifetimes. Consequently, grains probably reflect only a trace of their origin, although meteoritic inclusions with isotopic anomalies prove that some tiny particles survive intact from a supernova origin to the present. Grains apparently grow by coagulation while in clouds. Cometary and interplanetary dust suggest that very large sized grains are produced before extensive icy mantles are formed. Within the dark clouds, there is likely processing of the icy mantles by cosmic rays or the UV radiation produced by cosmic rays, and heavy molecules are released by runaway reactions. If there is an organic refractory mantle remaining after this processing, it is probably converted to almost pure amorphous carbon by the continued processing to which the grains are subjected.

There are several theories which explain the extinction law for diffuse dust, but a much more challenging problem is to understand the relation between dust of all types. The evolution of dust is probably the next theoretical challenge.

ACKNOWLEDGMENTS

This review has been partially supported by contract 957996 with the Jet Propulsion Lab and grant NAGW-1768 with NASA. Comments and assistance from L.J. Allamandola, J.A. Cardelli, G.C. Clayton, B.T. Draine, P.G. Martin, and B.D. Savage are appreciated.