Are Elias 2-27's spiral arms driven by self-gravity, or by a companion? A comparative spiral morphology study

D.H.F. gratefully acknowledges support from the ECOGAL project, grant agreement 291227, funded by the European Research Council under ERC-2011-ADG. J.D.I. and F.M. acknowledge support from the DISCSIM project, grant agreement 341137 under ERC-2013-ADG. F.M. also acknowledges support from The Leverhulme Trust, the Isaac Newton Trust, and the Royal Society Dorothy Hodgkin Fellowship. The authors warmly thank the anonymous referee for comments that helped to clarify the manuscript. This work used the Darwin DiRAC HPC cluster at the University of Cambridge, and the Cambridge COSMOS SMP system funded by ST/J005673/1, ST/H008586/1 and ST/K00333X/1 grants.The spiral waves detected in the protostellar disk surrounding Elias 2-27 have been suggested as evidence of the disk being gravitationally unstable. However, previous work has shown that a massive, stable disk undergoing an encounter with a massive companion are also consistent with the observations. We compare the spiral morphology of smoothed particle hydrodynamic simulations modeling both cases. The gravitationally unstable disk produces symmetric, tightly wound spiral arms with constant pitch angle, as predicted by the literature. The companion disk'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 disk 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 disk.Peer reviewe