The term "primordial" is used in reference to disks that are remnants of the star formation process. As outlined above, such disks are composed of the dust and gas which participated in the gravitational collapse that formed the star, and now comprise the raw materials for the formation of planets. The size, mass, and composition parameters of known young primordial disks are consistent with those estimated for the proto-solar system disk. Terrestrial planets and the rocky cores of giant planets originate from disk dust, while the gaseous envelopes of giant planets originate with the disk gas. Through either planet formation or one of the other disk dispersal mechanisms mentioned earlier, primordial disks are in the process of dissipating.
It is instructive to point out that primordial disks are physically distinguished from the so-called "debris" disks, which are secondary, rather than primordial. These gas-poor disks are comprised of dust which is regenerated during and subsequent to the growth of planets, as the large/massive bodies incite collisions amongst smaller bodies to re-form dust. Debris disks, like primordial disks, are in the process of dissipating, though via a different mechanism. Rather than sticking collisions which result in smaller particles growing to become larger particles (and eventually becoming undetectable via thermal infrared radiation), debris disk particles experience shattering collisions and gradually grind themselves down to the point at which grains are efficiently removed from the system via effects such as Poynting-Robertson drag and stellar winds. However, new dust is continually being generated in the cascade generated by collisions between the larger bodies, and the dissipating evolution is punctuated by the infusion of new material in the debris cascade.
Due to the influence of the outer giant planets on such debris, the collisional history in the inner solar system is well-documented in the cratering records on the Moon and Mars. To some degree, these records indicate the evolution of the cratering rate and the large-body size distribution with time. We have no firm record of the dust evolution in the Solar System, but even today there is "debris dust" found/assumed in the Asteroid and Kuiper belt regions. Because of the strong theoretical connection between debris dust and planetary perturbers, there is much interest in the debris belts seen around stars other than the Sun, whose evolution we can study by investigating samples of different age.
Here, I focus on the properties and evolution of dust in primordial disks. For any given disk, the dust mass is expected to decrease with time throughout the duration of the planet-building process, perhaps over tens of Myr. Then, if planets have successfully formed, the dust mass increases at the onset of the debris disk phase, before slowly declining again over many Gyr.
To understand the process of planet formation, we must understand how quantities such as: initial disk size and radial/vertical structure; initial disk mass and mass surface density; and initial disk composition and chemistry all evolve with time; and, further, the relative importance of various disk dispersal mechanisms (e.g., accretion, ablation, grain growth, as mentioned above). Over what timescales are dust (and gas) detectable, and how does the mass ratio of dust:gas evolve? What physical parameters determine disk longevity? What is the frequency of different end states, and in particular, that of planetary configurations? Most important for understanding the rarity or commonality of the formation of our own Solar System, what is the mean and the dispersion in all of the above distributions?
As we continue to develop the tools for answering these questions, we can also consider several pertinent "second parameter" categories. One of these relates to properties of the central star. Are there correlations in initial disk properties or disk evolution diagnostics with stellar properties such as the radiation field (particularly x-ray and ultraviolet output), stellar mass, or system metallicity, all of which may have important effects on disk structure and chemistry? A second category is related to disk physics effects. How does disk accretion history—in particular, poorly understood outburst phenomena such as FU Ori or EXOr type events—affect disk evolution? Thirdly, what is the role of environment? Multiplicity in the form of binary, triple, or quadruple systems can influence disk evolution when the companions are within, or just exterior to, the disk. Clustered versus isolated star-forming environments, in which effects such as increased ionization or photo-evaporation of disk material by massive stars, dynamical effects due to high stellar density, or the mechanical effects of multiple jets/outflows, could be important for disk evolution. Consequently, an understanding of multiplicity statistics in the form of frequency and orbital parameters, and clustering statistics in the form of spatial density and luminosity function is important for our understanding of the range of plausible disk-evolutionary paths.
In summary, there are many potentially influential parameters in the disk evolution process. The only way to effectively probe disk evolution and its many dependencies is through gathering sufficient statistics over the appropriate range of ages and "second parameter" conditions. This is a tall order indeed, but a road down which we have at least started.
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