In their youth, protoplanetary discs are expected to be massive and self-gravitating, which results in non-axisymmetric spiral structures. However recent observations of young protoplanetary discs with ALMA have revealed that discs with large-scale spiral structure are rarely observed in the midplane. Instead, axisymmetric discs with some also having ring & gap structures are more commonly observed.
To investigate this discrepancy between theoretical expectations and observation, we use 3D SPH simulations to consider two processes that are expected to commonly occur in these discs. The first is planet-disc interactions which is routinely used to explain the origin of the rings & gaps. The second is warps which can be caused by discs interacting in their chaotic star forming environment. Both of these processes are likely to occur when the disc is young, and potentially massive enough to be gravitationally unstable.
Our simulations show how the evolution of gravitationally unstable discs can be altered, potentially resolving this discrepancy by showing that massive discs that would be expected to be gravitationally unstable can appear axisymmetric when complex processes such as planet-disc interactions or warps are considered. Thus, the absence of observed large-scale spiral structures alone is not enough to place upper limits on the disc's mass.