Introduction

A long-standing paradigm for the formation of stars, and subsequently planets, involves the rotating collapse of a molecular cloud core to form a central proto-star surrounded by an infalling envelope and accreting disk on a timescale of ~105 yr. Typical ages of revealed young T Tauri and Herbig Ae/Be stars are ~106 yr. Gradual dispersal of the initially optically thick circumstellar material occurs in the early pre-main sequence phase as the system evolves through the final stages of disk accretion, which can last ~107 yr or more in at least some well known cases (TW Hya, Hen 3—600, TWA 14—Muzerolle et al. 2000, 2001 and Alencar & Batalha 2002; PDS 66—Mamajek et al. 2002; ECha J0843.3—7905—Lawson et al. 2002; St 34—White & Hillenbrand 2005).

Physical processes occurring in younger disks include viscous accretion onto the central star, mass loss due to outflow, irradiation by the central star, ablation due to the stellar wind, turbulent mixing of material, stratification, and gradual settling of the dust towards the disk mid-plane—this last process a critical and limiting step in the path towards planet formation in the standard core accretion model (e.g., Weidenschilling et al. 1997, 2000; Pollack et al. 1996). The total disk mass decreases and the dust:gas mass ratio, assumed at least initially to be in the interstellar ratio, changes with time due to a combination of the above effects. Similarly, the dust particles are assumed to be interstellar-like in their composition and structure. Of particular interest here is the expected loss of dust opacity due to assembly of small particles into larger bodies that

ACCRETION PHASE

POST-ACCRETION

DEBRIS DISKS

terrestrial planet formation J-----?

ACCRETION PHASE

POST-ACCRETION

DEBRIS DISKS

terrestrial planet formation J-----?

Figure 1. Images of disks at various evolutionary stages scaled to a timeline showing our general understanding of the basic phenomena. Data are courtesy of J. Stauffer and B. Patten (left panel, Ori 114-426 optically thick "silhouette disk" as imaged with HST/WFPC), Kalas & Jewitt (1995; middle panel, ( Pic as imaged by a ground-based coronagraph), and P. Kalas (right panel, our own zodiacal dust disk along with a comet, as photographed from Calar Alto).

might later be known as planetesimals. For solar-type stars, the ultimate result in at least 10%—and perhaps as many as 50%—of cases is a mature solar system (Marcy; this symposium).

In parallel with the discovery and study of exo-solar planets and planetary systems over the last decade (the topic of this conference), we have had dramatic observational confirmation in this same time period of the basic paradigm for star formation as briefly outlined above. Direct images and interferometric observations which spatially resolve young circumstellar disks at optical, near-infrared, and millimeter wavelengths have become common, though are far from ubiquitous. When combined with measured spectral energy distributions, such spatially resolved data are valuable for breaking model degeneracies and thus improving our understanding of source geometry and dust characteristics.

Rough correlation of the spatially resolved and SED appearances of a source, which indicate circumstellar status, with stellar evolutionary state, or age, has long been advocated (e.g., Lada 1987). However, it remains unclear whether the established sequence of circumstellar evolutionary states corresponds directly with source age. White & Hillenbrand (2004) argue for the Class I/II stages that this is not necessarily the case, given the similarities in the stellar photospheric and accretion properties of Class I and II stars as inferred from high dispersion spectroscopy of a large sample in Taurus-Auriga. Likewise, Kenyon & Hartmann (1995) discuss the Class II/III distribution in the HR diagram, which is indistinguishably intermingled, and therefore suggestive of similar ages. Because of uncertainties in age assignments, particularly for the most enshrouded sources which typically do not have ages estimated independent of their circumstellar characteristics, the timescales associated with the dispersal of circumstellar material and the formation of planets are only vaguely constrained at best.

How, then, do we catalog young circumstellar disks and characterize their evolution? Disk diagnostics come in two forms: those that trace the dust, and those that trace the gas. Dust implies small particles with typical tracers sensitive to sizes less than a mm.

These include continuum spectral energy distributions over several decades in wavelength, solid state spectroscopic features in the mid-infrared, and direct images measuring either thermal emission at long wavelengths (mid-infrared through millimeter) or scattered light at shorter wavelengths (optical and near-infrared). Gas tracers should reveal the bulk of the mass—at early stages, more than 99% of the total mass if interstellar abundances can be assumed. Sensitive gas observations of young circumstellar disks are, however, limited thus far, restricted to trace species, and dominated by upper limits. Yet recent observations of CO, H2, and H2O seem promising for characterization of terrestrial zone gas. Najita (this volume) presents our knowledge of gas disk evolution in detail.

In addition to academic interest in disk dissipation mechanisms, the main motivation for understanding disk evolution timescales is the relation to planet formation. It seems prudent then to begin with a summary of the capacity of young disks to form planets. We will then continue with methods for assessing the probability that young disks do indeed form planets.

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