Dispersal of protoplanetary disks

The lifetime of protoplanetary disks determines the time available for planet formation with the loss of the dusty gas disks no raw material is left to form planetesimals or giant planets. Thus, disk mass as a function of time is perhaps the single most important constraint on the formation of both the rocky and the giant planets. The most readily observable, albeit imperfect, indicator of disks is the presence of excess emission above the stellar photosphere, emerging from small, warm dust grains. 

Observations of near-infrared excess emission from hundreds of disks with ages covering the first 10 Myr demonstrate fundamental structural evolution and the eventual loss of the fine dust from the inner disk ( 1AU). The declining fraction of stars with dust disks suggests a disk half-life of 3 to 5 Myr (see Chapter 9, e.g. Hernandez et al. 2007). Longer-wavelength infrared observations, primarily from the Spitzer Space Telescope, show a similar picture for the intermediate disk radii (1-5 AU). The combination of these lines of evidence is interpreted as a rapid ( 1-3 Myr) dispersal of the fine dust in most systems, probably progressing inside-out. 

Although the disk mass is dominated by hydrogen, much less is known about its dispersal. Tracers of hot gas in the innermost disk regions show a one-to-one correspondence to the presence of hot dust (Harfigan et al. 1995) and gas accretion to the stars declines at the same rate as hot dust disperses. Spitzer studies of mid-infrared ro-vibrational lines probe warm gas on orbits similar to Jupiter s and demonstrate the loss of gas in few tens of millions of years (Pascucci et al. 2007). Gas in the coldest disk regions can be traced through CO rotational lines such studies also suggest a gas depletion by 10 Myr. The combined astronomical evidence shows that (1) dust disks dissipate in 3—8 Myr via rapid inside-out dispersal (2) gas dissipates in a similar, or perhaps even shorter timescale. 

The Solar System, in comparison, offers two lines of evidence to constrain the timescale for the lifetime of the proto-solar nebula and the epoch of planetesimal formation. On one side, relatively unaltered chondritic components preserve traces of their chemical and thermal history on the other side, dynamical information is imprinted on the hierarchy of the Solar System. 

The observation that bulk chondrites are isotopically homogeneous - with the exception of H, C, N, and O - is evidence for a very thorough mixing in an early phase in the hot nebula ( 2000 K). Chondrules themselves provide a variety of constraints on the dust and gas content of their natal environment. Their chemical and isotopic compositions, size, and shape distributions all suggest dusty gas reservoirs during their formation epoch, which lasted 1-3 Myr after CAI formation, possibly with a peak at 2 Myr. 

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Observations of protoplanetary disks indicate that these objects remain optically thick for timescales of millions of years, meaning that a population of dust is sustained for that period of time (see Chapter 9 for a detailed discussion of disk lifetimes and dispersal mechanisms). As will be discussed in Chapter 10, the timescale for dust growth and incorporation into planetesimals is less than this time period. Additionally, the timescale for dust settling is much less than the age of these disks. However, the apparent contradictions between these timescales and the observations can be explained within the context of the processes described thus far. 

In this chapter we compare the evolution of protoplanetary disks to that of the proto-solar nebula. We start by summarizing the observational constraints on the lifetime of protoplanetary disks and discuss four major disk-dispersal mechanisms. Then, we seek constraints on the clearing of gas and dust in the proto-solar nebula from the properties of meteorites, asteroids, and planets. Finally, we try to anchor the evolution of protoplanetary disks to the Solar System chronology and discuss what observations and experiments are needed to understand how common is the history of the Solar System. 

Studies of the gas content of protoplanetary disks with ages between 1 and 30 Myr are necessary to determine how rapidly the gas disperses and make a more direct comparison to the evolution and dispersal of dust in disks. As we discussed in Section 9.1.2, the dispersal of gaseous disks also provides an upper limit for the formation time of giant planets that can be compared to the time necessary to form Jupiter and Saturn in our Solar System. From a Solar System perspective it is interesting to expand on the constraints placed on the gas dispersal from the age determination of meteorites with implantation of solar wind, which provide us a... 

To summarize, the evolution and dispersal of the dust in the proto-solar nebula seem to have followed a path similar to that of dust in most protoplanetary disks if CAIs formed at around 1 Myr in the disk evolutionary frame. If they formed... 

After a few million years of evolution, where most of the remaining disk mass is accreted to the T Tauri star, the residual disk is dispersed (see Chapter 8) and the star continues its further evolution to the main sequence as a Class III object. The processes going on in these disks, that are usually called protoplanetary during this final phase of protostar evolution, are the subject of the following chapters of this book. 

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