|Annu. Rev. Astron. Astrophys. 1990. 28:
Copyright © 1990 by Annual Reviews. All rights reserved
Relevant timescales show that the present form of interstellar dust must be more a reflection of the processing it has received within the ISM than of the conditions at its origin. A typical parcel of gas and dust is cycled back and forth through molecular clouds several times during its lifetime, changing its grain properties significantly each time.
The lifetime of a grain against incorporation into stars can be estimated by dividing the surface density of the ISM by the rate of star formation. The local H I surface density is ~ 107 M kpc-2 (89), and for H2 is 3 x 106 M kpc-2 (147). A mean rate of star formation of 3.4 x 10-3 M kpc-2 yr-1 would account for the present surface density of low-mass (M M) stars over 1010 yr (6); presumably the present rate is lower. High-mass stars contribute about 1.1 M kpc-2 yr-1 (135). The corresponding mean lifetime of a parcel of gas/dust in the ISM is more than 3 Gyr. On the other hand, about 30% of the local ISM is in molecular clouds, each with a lifetime of perhaps 108 yr (the time for the gas to proceed from one spiral arm to the next) or less. These numbers imply that a given parcel of gas has been into and out of a molecular cloud at least every 3 x 108 yr, or more than 10 times during its mean lifetime. Each time, the differences in extinction laws between diffuse dust and inner-cloud dust require that the grains be heavily modified.
Let us follow the state of a typical parcel of gas/dust near the Sun (see also 39). Since most of the mass of the local ISM is in H I, the parcel must spend the bulk of its time outside of molecular clouds. During this phase, about 10% of its mass is returned to the ISM from stars. Perhaps 10-20% of the returned gas is from hot stellar winds with no dust, or from planetary nebulae with a dust/gas ratio lower than the ISM, providing substantial amounts of gaseous Fe, Al, and the other strongly depleted elements (81). Refractory elements must encounter grains frequently and stick to them very efficiently in order to achieve the observed strong depletions in the denser parts of the diffuse ISM.
Each time the gas/dust mixture is incorporated into the outer regions of a molecular cloud, many things happen to the grains: (a) The extinction law changes from diffuse dust to outer-cloud dust in such a way that the relative numbers of small, medium, and large grains depend primarily upon only one parameter (local gas density?), regardless of the local environment or past history. (b) The refractory elements are more strongly depleted onto the grains than in the diffuse phase. (c) The atomic H becomes molecular. Gaseous carbon recombines from C+ to C0, and finally to CO. (d) The grains almost certainly coagulate in the outer parts of molecular clouds before there is much coating of icy mantles. Much deeper in the cloud, the fluffy micron-sized cometary and interplanetary dust particles can be produced by further coagulation of the silicate and carbonaceous parts of the grains, with ices probably filling the voids and producing the observed molecular absorption features. It is difficult for me to see how fluffy cometary mineral grains can form within the cloud if icy or organic refractory mantles envelop the minerals before the coagulation.
Coagulation seems to dominate the change in the size distribution in going from the diffuse ISM to outer-cloud dust. At least two well-observed stars in outer-cloud dust have A(V) / N(H) smaller than in diffuse dust (CCM89). Since [A(-1) / A(V)] d-1 is much lower in the outer-cloud dust than in diffuse dust (Figure 2), in these stars the integrated extinction per H atom is substantially smaller than in diffuse dust. This reduction in grain cross-section per H, in spite of the accretion of small amounts of the refractory elements, can only be achieved by coagulation, which prevents the inner parts of the larger grains from participating efficiently in the extinction. Adding substantial coatings would increase, rather than reduce, the extinction cross-section per H for the dust in the cloud.
There are real deviations of the various extinction laws about the mean. These differences must reflect somewhat different histories and present local environments (radiation field, shocks, magnetic fields, etc.) of the grains along each line of sight. Studying these deviations should lead to a better understanding of the factors which influence extinction laws.