Is the spiral morphology of the Elias 2-27 circumstellar disc due to gravitational instability?

Cassandra Hall,Ken Rice,Tim Harries,Duncan Forgan,Richard Alexander,Giovanni Dipierro

doi:10.1093/mnras/sty550

Abstract

A recent ALMA observation of the Elias 2-27 system revealed a two-armed structure extending out to ~300 au in radius. The protostellar disc surrounding the central star is unusually massive, raising the possibility that the system is gravitationally unstable. Recent work has shown that the observed morphology of the system can be explained by disc self-gravity, so we examine the physical properties of the disc necessary to detect self-gravitating spiral waves. Using three-dimensional Smoothed Particle Hydrodynamics, coupled with radiative transfer and synthetic ALMA imaging, we find that observable spiral structure can only be explained by self-gravity if the disc has a low opacity (and therefore efficient cooling), and is minimally supported by external irradiation. This corresponds to a very narrow region of parameter space, suggesting that, although it is possible for the spiral structure to be due to disc self-gravity, other explanations, such as an external perturbation, may be preferred.

Highlights

It has long been known that spiral structure exists in galaxies, with many examples of striking spiral arms, such as those found in the Pinwheel galaxy (M101) and the Whirlpool galaxy (M51).Despite many examples of spiral structure in numerically simulated protostellar discs present in the literature, these structures, unlike their galactic cousins, have only recently been clearly observed in nature, due to significant advances in imaging capability.One such advance has been SPHERE (Beuzit et al 2008), the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument mounted on UT3 at the VLT
We have simulated a range of disc masses and metallicities for the Elias 2-27 system, and we conclude that only in the case of the most efficient cooling, is nonaxisymmetric structure visible using the same observational settings as in the original detection, without the need to employ an enhancement of the spirals
We present the results of a suite of Smoothed Particle Hydrodynamics (SPH) simulations investigating if the morphology of the Elias 2-27 system could be due to gravitational instability

Summary

Introduction

Boss 1989, 1998; Gammie 2001; Rice et al 2003a), these structures, unlike their galactic cousins, have only recently been clearly observed in nature (see, e.g., Garufi et al 2013; Pérez et al 2014; Benisty et al 2015; Stolker et al 2016), due to significant advances in imaging capability One such advance has been SPHERE (Beuzit et al 2008), the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument mounted on UT3 at the VLT. Since the Atacama Large Millimeter/submillimeter array (ALMA) began operations, the astronomical community has, for the first time, been able to resolve midplane structure in protostellar discs. With this capability has come an array of unexpected results - these discs are often far from smooth, and a plethora of substructures have been revealed. It is possible that these rings are caused by planets carving gaps in the dust of the disc (Dipierro et al 2015b; Jin et al 2016), but these features have plausible alternative explanations, such as aggregate sintering (Okuzumi et al 2016), particle trapping at the edge of the disc dead-zone (Ruge et al 2016) and particle concentration at planet-induced gap edges (Zhu et al 2014), to name a few

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