  The spiral waves detected in the protostellar disc surrounding Elias 2-27
have been suggested as evidence of the disc being gravitationally unstable.
However, previous work has shown that a massive, stable disc undergoing an
encounter with a massive companion are also consistent with the observations.
We compare the spiral morphology of smoothed particle hydrodynamic simulations
modelling both cases. The gravitationally unstable disc produces symmetric,
tightly wound spiral arms with constant pitch angle, as predicted by the
literature. The companion disc's arms are asymmetric, with pitch angles that
increase with radius. However, these arms are not well-fitted by standard
analytic expressions, due to the high disc mass and relatively low companion
mass. We note that differences (or indeed similarities) in morphology between
pairs of spirals is a crucial discriminant between scenarios for Elias 2-27,
and hence future studies must fit spiral arms individually. If Elias 2-27
continues to show symmetric tightly wound spiral arms in future observations,
then we posit that it is the first observed example of a gravitationally
unstable protostellar disc.
