Dynamics of cold circumstellar gas in debris discs

Mounting observational evidence indicates that cold circumstellar gas is present in debris disc systems. This work focuses on various dynamical processes that debris disc gas may undergo. We review five mechanisms that can transport angular momentum and their applications to debris discs. These include molecular viscosity, hydrodynamic turbulence, magnetohydrodynamic (MHD) turbulence, magnetized disc winds, and laminar magnetic stress. We find that molecular viscosity can result in alpha as high as less than or similar to 0.1 for sufficiently low densities, while the Rossby wave instability is a possible source of hydrodynamic turbulence and structure formation. We argue that the vertical shear instability is unlikely due to the long cooling times. The onset of the magnetorotational instability (MRI) is dichotomous: for low-density discs the MRI can be excited at the mid-plane, while for high-mass discs it may only be operating at z > 2 - 3H, if at all. The MHD wind and laminar magnetic stress mechanisms rely on the configuration and strength of any background large-scale magnetic field, the existence of which is uncertain and possibly unlikely. We conclude that the dominant mechanism and its efficiency in transporting angular momentum vary from one system to the other, depending especially closely on the gas density. More detailed analyses shall be performed in the future focusing on representative, nearby debris discs.