Speaker
Description
The evolution of protoplanetary disks is set by the conservation of angular momentum, where the accretion of material onto the central star is balanced by viscous expansion of the outer disk or by disk winds extracting angular momentum without changing the disk size. Studying the time evolution of disk sizes allows us therefore to distinguish between viscous stresses or disk winds as the main mechanism of disk evolution. Observationally, estimates of the disk gaseous outer radius are based on the extent of the CO rotational emission, which, during the evolution, is also affected by the changing physical and chemical conditions in the disk.
We have used physical-chemical models to study how the extent of the CO emission changes with time in a viscously expanding disk. We find that the gas outer radius ($R_{\rm CO,\ 90\%}$) measured from our models matches the expectations of a viscously spreading disk: $R_{\rm CO,\ 90\%}$ increases with time and for a given time $R_{\rm CO,\ 90\%}$ is larger for a disk with a higher viscosity $\alpha_{\rm visc}$. However, in the extreme case where the disk mass is low ($ \leq 10^{-4}\ \mathrm{M}_{\odot}$) and $\alpha_{\rm visc}$ is high ($\geq 10^{-2}$), $R_{\rm CO,\ 90\%}$ will instead decrease with time as a result of CO photodissociation in the outer disk.
We find that most observed gas outer radii in Lupus can be explained using a viscously evolving disk that starts out small $(\simeq 10\ \mathrm{AU})$ and has a low viscosity $(\alpha_{\rm visc} = 10^{-4} - 10^{-3})$.
