Signatures of an eccentric disc cavity: Dust and gas in IRS 48

TL;DR

This study uses 3D simulations to show that a low-mass binary companion can create a cavity and dust asymmetries in IRS 48's disc, explaining observed features and velocity asymmetries without invoking a vortex.

Contribution

It demonstrates that a low binary mass ratio can produce observed disc features and velocity asymmetries, challenging the vortex hypothesis in IRS 48.

Findings

Dust concentrates in gas over-density at cavity edge.

Observed velocity asymmetries are explained by non-Keplerian flow in an eccentric cavity.

A ~0.4 M_sun binary at 10 au can reproduce the observed features.

Abstract

We test the hypothesis that the disc cavity in the `transition disc' Oph IRS 48 is carved by an unseen binary companion. We use 3D dust-gas smoothed-particle hydrodynamics simulations to demonstrate that marginally coupled dust grains concentrate in the gas over-density that forms in in the cavity around a low binary mass ratio binary. This produces high contrast ratio dust asymmetries at the cavity edge similar to those observed in the disc around IRS 48 and other transition discs. This structure was previously assumed to be a vortex. However, we show that the observed velocity map of IRS 48 displays a peculiar asymmetry that is not predicted by the vortex hypothesis. We show the unusual kinematics are naturally explained by the non-Keplerian flow of gas in an eccentric circumbinary cavity. We further show that perturbations observed in the isovelocity curves of IRS 48 may be explained as the product of the dynamical interaction between the companion and the disc. The presence of a $\sim$0.4 M$_{\odot}$ companion at a $\sim$10 au separation can qualitatively explain these observations. High spatial resolution line and continuum imaging should be able to confirm this hypothesis.