Dust coagulation

The ubiquitous presence of silicate emission features in young protoplanetary disks is evidence that a population of small (a few micron) particles persists on million year timescales, much longer than the grain coagulation timescales (e.g. Dullemond Dominik 2005 Brauer et al 2008). This demonstrates that an efficient mechanism must operate that replenishes particles in the 1-10 pm size range, at least in the upper layers of protoplanetary disks. 

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On a more local scale, the effects of turbulence are to impart random motions to particles that are then added to the motions that those particles experience owing to other effects. The details of the resulting motions are beyond the scope of this chapter (comprehensive discussion and equations are provided in Cuzzi Weidenschilling 2006 Ormel Cuzzi 2007). A characteristic velocity that describes these motions is the root-mean-square turbulent velocity, /acs which is the overturn velocity of the largest eddies, and an estimate of the maximum velocity two particles (both of St = 1) would develop with respect to one another (Cuzzi Hogan 2003). Under nominal conditions, such velocities can exceed 100 m s-1 (for a = 0.01). These velocities are important to consider when developing models for dust coagulation and planetesimal formation (see Section 3.4.1 and Chapters 7 and 10). 

The focus of the chapter is on the smallest range of particle sizes (< 1 cm) and thus the initial phases of dust coagulation in young gas-rich disks, since these are the ones that can be observed in extrasolar systems and have preserved some very early record in chondritic meteorites and interplanetary dust particles (IDPs). 

Dust coagulation in the Solar System and in extrasolar protoplanetary disks... 

What is considered evidence for protoplanetary dust coagulation ... 

Dependencies of grain composition, e.g. icy grains versus silicates or carbon, as well as their charge state, are also of interest, and for destructive collisions, the resulting fragment size distribution is important (Wurm et al. 2005). These are dependent on the microphysics of the adhesive forces between monomers (Chokshi et al. 1993 Blum Schrapler 2004). A review of laboratory simulations of dust coagulation can be found in Blum Wurm (2008). 

In order to consider the processes of dust coagulation in the early Solar System, we first review the characteristics of this material. Of considerable importance is the fact that these samples - represented principally by chondritic meteorites, but also by IDPs and by samples from Comet Wild 2 collected by the Stardust mission - all come from parent bodies of different kinds. As a result, even the most primitive of these materials has been processed, both physically and chemically, to different degrees. The processes that affected Solar System dust may have occurred in different environments such as the solar nebula (e.g. evaporation/condensation, annealing) and asteroidal parent bodies (aqueous alteration and/or thermal processing, mild compaction to extensive lithihcation). A major challenge is to understand the effects of this secondary processing. 

A key question in addressing the issue of dust coagulation concerns the grain size of dust. This is closely linked to the formation mechanisms of the dust as discussed above. It is important to note that larger grains reaching sizes of up to 10 pm (e.g. Fig. 7.6a,b) do occur in the matrices of primitive chondrites. However, these grains typically represent <1 vol% of primitive matrix and hence are a very minor component. 

Constraints on dust coagulation from amorphous silicates... 

Henning, T., Dullemond, C. P, Wolf, S., Dominik, C. 2006, Dust Coagulation in Protoplanetary Disks, Cambridge University Press. 

Dominik C, Helens AGGM (1997) The physics of dust coagulation and the structure of dust aggregates in space. Astrophys J 480 647... 

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