Predicting the kinematic evidence of gravitational instability
  Observations with the Atacama Large Millimeter/Submillimeter array (ALMA)
have dramatically improved our understanding of the site of exoplanet
formation: protoplanetary discs. However, many basic properties of these discs
are not well-understood. The most fundamental of these is the total disc mass,
which sets the mass budget for planet formation. Discs with sufficiently high
masses can excite gravitational instability and drive spiral arms that are
detectable with ALMA . Although spirals have been detected in ALMA observations
of the dust , their association with gravitational instability, and high disc
masses, is far from clear. Here we report a prediction for kinematic evidence
of gravitational instability. Using hydrodynamics simulations coupled with
radiative transfer calculations, we show that a disc undergoing such
instability has clear kinematic signatures in molecular line observations
across the entire disc azimuth and radius which are independent of viewing
angle. If these signatures are detected, it will provide the clearest evidence
for the occurrence of gravitational instability in planet-forming discs, and
provide a crucial way to measure disc masses.
