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

TL;DR

This study compares the spiral structures in Elias 2-27's protostellar disc to determine if they are caused by gravitational instability or a companion, using simulations to identify distinctive morphological features.

Contribution

It provides a comparative analysis of spiral morphologies from simulations of both scenarios, highlighting key differences for observational discrimination.

Findings

Symmetric, tightly wound spirals suggest gravitational instability.

Asymmetric spirals with increasing pitch angles indicate a companion influence.

Future observations of spiral symmetry can identify the dominant formation mechanism.

Abstract

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.