From a theoretical perspective, several processes are expected to govern the evolution of gaseous disks. These include viscous accretion, which is responsible not only for the radial spreading of disks, but also the draining of the disk onto the central star. The latter effect may play an important role in the dissipation of the inner disk region (<5-10 AU). Another potentially important dissipation process is photoevaporation, where energetic photons from the central star heat the disk surface, causing it to become dynamically unbound from the system and to flow off the disk surface in a photoevaporative flow (e.g., Hollenbach et al. 2000). Photoevaporation has been suggested as an important dissipation mechanism for the outer disk region (>5-10 AU; Shu et al. 1993; Clarke et al. 2001). Finally, the planet formation process itself may play a fundamental role in dissipating gaseous disks.
From an observational perspective, little is known directly about the evolution of the gaseous component of the disk. Most of what we know comes instead from surrogate diagnostics such as emission from circumstellar dust (as measured by IR excess fraction— e.g., Haisch et al. 2001; see also Hillenbrand, this volume) and stellar accretion rates (e.g., Sicilia-Aguilar et al. 2005). Both of these show a significant decline in the 1-10 Myr range, results that are interpreted as indicating the dissipation of the gaseous disk. However, both diagnostics have important caveats associated with their use as a probe of the evolution of gaseous disks. For stellar accretion rates, we might want to know how to convert a measured stellar accretion rate into a gas column density, a topic that I will return to in Section 4. For the IR excesses, we might wonder whether the decline in IR excess as a function of age is primarily the result of disk dissipation or possibly a signature of grain growth. That is, could grain growth, an important first step in the planet formation process, render inner disks optically thin in the continuum, leaving behind a significant reservoir of gas?
Another caveat associated more specifically with near-infrared excess fractions as a function of age (e.g., Haisch et al. 2001) is that they probe the very inner region of the disk AU. So any evolution that might be deduced for this region of the disk may not apply at larger radii. To address this concern, people have typically turned to the measurement of continuum excesses at longer wavelengths, at mid-infrared (e.g., Skrutskie et al. 1990; Mamajek et al. 2004; Silverstone et al. 2006) and submillimeter to millimeter (e.g., Osterloh & Beckwith 1995; Duvert et al. 2000; Andrews & Williams 2005) wavelengths. The latter have also been used to estimate disk masses. However, the possibility that the dust component of the disk experiences collisional and dynamical evolution on 1-10 Myr timescales suggests that some caution may be needed in interpreting these results.
Several physical processes beyond grain growth may alter the gas-to-dust ratio in different parts of the disk, thereby compromising the ability of the dust to track the evolution of the gaseous component in a simple way. For example, in the mid- to far-infrared, collisions between planetesimals may create a new population of small dust grains, decreasing the gas-to-dust ratio. Indeed, terrestrial planet formation is believed to involve planetesimal and protoplanet collisions at ages of 1-10 Myr—collisions which are expected to generate observable collisional debris (Kenyon & Bromley 2004). As a result, infrared excesses may be detected, even from systems with highly depleted gaseous disks. At longer wavelengths, it is well known that the submillimeter-to-millimeter spectral slopes generally indicate that grains have grown to millimeter sizes or larger at distances of ^100 AU in Myr-old disks. Takeuchi & Lin (2005) show that gas drag is capable of causing such large grains to lose angular momentum and spiral into the star on Myr timescales. Thus, a reduction in submillimeter excess on Myr timescales may not rule out the survival of massive gaseous disks at large radii. These caveats suggest that using direct probes of the gaseous component of disks can provide useful, possibly critical information on the evolution of gaseous disks.
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