Protoplanetary Disk Structure

The protoplanetary disks, or circumstellar disks around young stars, have a very rich structure, made up of several regions where different physics apply and hence different wavelength ranges need to be observed. For this reason, the techniques used to observe these regions vary with the characteristic temperatures and scale sizes involved. 

A protoplanetary disk contains gas and dust. The chemistry is dominated by freeze out and low temperature ion-molecule reactions. Outflows and UV radiation clear out most of the envelope, leaving a protoplanetary disk with the inner gap between the disk and the star. Close to the dust inner rim temperatures are high and X-rays heat dust and gas, which in turn excite molecular lines such as Ha, fine-structure lines, and HaO, among others. The middle planes are cold and gas is frozen out on dust grains. The outer disk is irradiated by UV light from the star showing mostly molecular rotational lines in the form of ions and radicals. 

The thermal emission from the protoplanetary disks can be observed, as well as the scattered light from the dust grains in the surfaces of these disks which is comparable to the light received from stars. This disk radiation appears in the spectral energy distribution of a young star as an excess of infrared radiation. 

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Abstract In this chapter, we review the general properties of protoplanetary disks and how the gaseous and solid components contained within evolve. We focus on the models that are currently used to describe them while highlighting the successes that these models have had in explaining the properties of disks and primitive materials in our Solar System. We close with a discussion of the open issues that must be addressed by future research in order to develop fully our understanding of protoplanetary disk structures. 

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