Another interesting population of young stars are weak T Tauri stars. These sources are accreting at low rates, if at all, and typically show no signs of infrared excess. They make up a significant fraction of T Tauri stars (~50%) and span a wide range of ages, from those comparable to those of the youngest classical T Tauri stars to those >10 Myr (Kenyon & Hartmann 1995). Thus, these sources show signs of neither circumstellar dust nor gas, as might be deduced from stellar accretion rates.
What might be the nature of these sources? Weak T Tauri stars may represent sources with very small initial disk masses, disks that dissipated rapidly and were possibly too low in mass to form any planets. These might, then, be failed planetary systems. They might also represent sources with very large initial disk masses, disks that went gravitationally unstable early on, thereby forming a planetary system and dissipating the disk (e.g., Mayer et al. 2002). Since the gravitational instability mode of planet formation works quickly, this scenario may plausibly explain the very youngest weak T Tauri stars.
Alternatively, weak T Tauri systems might have disks in which grain growth to millimeter sizes and the consequent strong dynamical interaction between the gas and grains in the disks has resulted in the rapid inspiral of the dust component, leaving behind a potentially large reservoir of gas. If grain growth has also led to the formation of plan-etesimals, then these might be systems in the process of giant planet formation.
In summary, the possibilities range the entire gamut in terms of the possible outcome of planet formation in these systems.
How might we distinguish between these possibilities? Systems that have successfully formed giant planets through gravitational instabilities could be identified by the presence of massive, distant planetary companions. Searches for low-mass companions to weak T Tauri stars in the >150 AU separation range detect potential companions at large separations (>500 AU) in some weak T Tauri systems (Massarotti et al. 2005). Sensitive companion searches at smaller radii <150 AU will help to determine whether planet formation via gravitational instabilities is a plausible explanation for the properties of weak T Tauri stars. To address the possibility that weak T Tauri stars are systems in which grain growth and subsequent inspiral has occurred under the action of gas drag, we might search for the presence of large gaseous reservoirs using any of the diagnostics discussed in Section 4.
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