The dynamics of disk formation
How do planets form in accretion disks? What is the origin of structure leading to planet formation? One possibility, but not the only possibility, is gravitational fragmentation of an unstable disk. Under what conditions does this take place? Do all disks fragment?
The current observational state of the art for low-mass stellar systems is represented by recent ALMA, Subaru, VLTI, and VLA observations of transitional disks in the dust continuum showing gaps consistent with clearing by planets (TW Hydra: Menu 2014; Akiyama et al. 2015; HD100546: Wright et al. 2015; and a small sample of other disk systems: van Dishoeck et al. 2015).
In contrast, observations of higher mass systems reveal processes at an earlier stage in the formation of the disk itself where gravitational instabilities would first develop. Our recent observations of the accretion disk around IRAS20126, an approximately 10 solar mass star, show how the CH3CN spectral line may be used to measure the disk density, shear velocity, and thermal and non-thermal energy. These are all factors in the Toomre-Q parameter and the Shakura-Sunyaev alpha parameter which allow us to estimate the stability of the disk against internal fragmentation (Chen, Keto et al., submitted).
In this talk, I will compare observations of low and higher mass systems to show how we can expect to directly measure disk stability within the next cycle of ALMA observations.