Protoplanetary Disk Evolution

Originally, the protoplanetary disk contains gas and dust with a composition similar to the parental molecular cloud. During the course of evolution, this material turns into larger bodies such as comets, asteroids, and planets. Because these disks are very opaque in their youngest phase, it is difficult to observe this process directly. However, the stellar radiation is absorbed by the gas and dust in the disk and heats the matter to typical temperatures of a few 1000 K in the inner disk regions to 10 K in the outer regions. 

After about one million years (for solar-mass stars, this process is much faster for higher masses), the combination of outflow and infall disperses the majority of the envelope and the star is optically revealed, although a circumstellar disk is still present. For solar-mass stars, this is the T Tauri phase, while for intermediate masses, these stars are referred to as Herbig Ae/Be stars (Hillenbrand et al. 1992). Several million years after the primordial disk has almost disappeared. 

Disks are generally associated with the Class 11 spectral energy distributions (SED) from the pre-main-sequence (PMS) infrared spectral classification of Young Stellar Objects (YSO) (Lada 1987). This initial classification was based on whether the emitted energy from YSOs was rising in the mid-IR or with negligible IR excess. 

Currently there are four classes to the PMS classification. Classes 0,1, n and III spectra are seen as an evolutionary sequence with Class 0 being the youngest sources embedded in natal material, the collapse phase. Class I corresponds to the accreting protostar. Class II is the stage when the properties of the disks and their central stars first become optically visible. Finally, Class III YSO spectra represents the dissipation of circumstellar material prior to the protostar reaching the main sequence. 

However, it must be noticed that SED classification does not give a unique description of the amount and distribution of circumstellar material. For instance, when a YSO has a highly extinct edge-on disk it can be mis-interpreted as a more embedded, hence, less evolved object. 

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The fundamental initial parameters of protoplanetary disk evolution are the masses and sizes of the disks. Optical silhouettes of disks in the Orion Nebula Cluster (McCaughrean O Dell 1996), scattered light imagery (e.g. Grady et al. 1999), interferometric maps in millimeter continuum or line emission (e.g. Rodmann et al. 2006 Dutrey et al. 2007), and disk spectral energy distributions... 

Thus far we have introduced and provided brief overviews of the dynamical effects believed to be responsible for shaping the environments in which dust would be processed within protoplanetary disks. In the remaining part of this chapter we discuss how these processes may have left their fingerprints on the properties of dust, as revealed from telescopic observations and from laboratory analysis of primitive samples. This should not be taken as a definitive discussion, but rather as an examination of how current models of protoplanetary disk evolution can be used to make sense of the properties of meteorites, comets, and the dust that is present around young stars. 

With the success of the Stardust mission, tests of our models for solar nebula, and thus protoplanetary disk evolution, are no longer limited to asteroidal bodies (meteorites), but now can be applied to cometary bodies as well. Stardust returned dust grains that were ejected from the surface of comet Wild 2, a Jupiter-family cometthatis thought to have formed at distances of >20 AU from the Sun (Brownlee et al. 2006). Thus, we now have samples of materials from the outer solar nebula that can be studied in detail. 

Gammie, C. F. and Johnson, . M. (2005) Theoretical studies of gaseous disk evolution around solar mass stars. In Chondrites and the Protoplanetary Disk, ASP Conference Series, 341, eds. Krot, A.N., Scott, E. R. D. and Reipurth, A. San Francisco Astronomical Society of the Pacific, pp. 145-164. 

Tiny solid cosmic particles - often referred to as dust - are the ultimate source of solids from which rocky planets, planetesimals, moons, and everything on them form. The study of the dust particles genesis and their evolution from interstellar space through protoplanetary disks into forming planetesimals provides us with a bottom-up picture on planet formation. These studies are essential to understand what determines the bulk composition of rocky planets and, ultimately, to decipher the formation history of the Solar System. Dust in many astrophysical settings is readily observable and recent ground- and space-based observations have transformed our understanding on the physics and chemistry of these tiny particles. 

Table 1.1 The astronomical and cosmochemical evidence available on the key stages of the evolution of protoplanetary disks and the chapters in which they are discussed.

The collapse of rotating molecular cloud cores leads to the formation of massive accretion disks that evolve to more tenuous protoplanetary disks. Disk evolution is driven by a combination of viscous evolution, grain coagulation, photoevaporation, and accretion to the star. The pace of disk evolution can vary substantially, but massive accretion disks are thought to be typical for stars with ages < 1 Myr and lower-mass protoplanetary disks with reduced or no accretion rates are usually 1-8 Myr old. Disks older than 10 Myr are almost exclusively non-accreting debris disks (see Figs. 1.3 and 1.5). 

In order to understand fully the evolution of protoplanetary disks, it is necessary to develop an understanding of the dynamics and evolution of both the gas and solids that they contain. The detailed evolution of these components are often discussed separately, largely owing to our incomplete understanding of the processes involved. In introducing the different concepts in this chapter, we too will treat these components separately in order to present a clear picture of the fundamental processes that are believed to be at work. We will discuss the feedback that takes... 

The masses, sizes, and overall structure of protoplanetary disks are important to quantify as they set the total amount and the distribution of planet-forming materials. However, over time disks evolve and the dust contained within is transported, processed, and accreted into larger bodies. This evolution plays a critical role in determining both the physical and chemical properties of the dust, and by extension, of the planets that will eventually form. 

While the viscous model for the evolution of protoplanetary disks has had some success in matching some of the general properties of protoplanetary disks, such as the observed mass accretion rates and effective temperatures, the exact source of the viscosity remains the subject of ongoing studies. Currently, the most popular candidates for driving the mass transport in protoplanetary disks are the magneto-rotational instability (MRI) and gravitational instability. A third candidate, shear instability, has also been proposed based on laboratory experiments of rotating fluids (Richard Zahn 1999), but questions remain as to whether these results can be extended to the scale of protoplanetary disks. 

In short, there is still much to learn about the specific processes that drive disk evolution. Currently, the two best candidates, the MRI and gravitational torques, likely produce values of a that vary with time and location in a protoplanetary disk. In order to account properly for these variations more detailed models than those discussed in this chapter are required, and due to their complexity and computational rigor, the amount of model time that could be investigated is limited to 103-104 yr. While adopting a constant or effective value of a overlooks the details of these variations, it greatly reduces the numerical complexity of disk models, allowing the evolution of disks for times > 106 yr to be calculated. This provides a way for the timescales that are needed to study meteoritic materials or dust around other stars to be modeled. 

Figure 3.4 Viscous evolution of a protoplanetary disk around a solar-type star. The initial disk radius is 30 AU and initial disk mass is 0.01 M . The EUV flux from the central star is 1041 photons per second.

The dynamical evolution of solids in protoplanetary disks is controlled by the large-scale flows that develop due to disk evolution, diffusion associated with the turbulence that is related to disk evolution or shear instabilities, gas-drag-induced motions due to different orbital velocities of the gas and solids, and settling towards the mid-plane due to the vertical component of the central star s gravity. As the large-scale flows have already been discussed, we now discuss the other sources of particle motions. 

The resulting transport of gas in an evolving protoplanetary disk is unlikely to have been perfectly ordered. Instead, the release of large amounts of gravitational energy associated with such evolution would likely manifest itself in small-scale fluctuations and random motions within the gas. These random motions represent the turbulence that is often discussed in terms of the evolution of protoplanetary disks, and they give rise to a diffusivity within the gas. 

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