Direct detection of precursors of gas giants formed by gravitational instability with the Atacama Large Millimetre/sub-millimetre Array

TL;DR

This paper demonstrates through simulations that ALMA can directly detect early gaseous clumps in protoplanetary disks, providing evidence for the disk instability model of gas giant formation.

Contribution

It presents the first detailed simulation-based analysis showing ALMA's capability to detect precursors of gas giants formed by gravitational instability.

Findings

ALMA can detect overdense spiral arms and collapsing clumps in disks at 125 pc.

Detection is most effective at 690 GHz with multi-scale clean.

Estimated clump masses from flux can be within a factor of 3 of true masses.

Abstract

Phases of gravitational instability are expected in the early phases of disk evolution, when the disk mass is still a substantial fraction of the mass of the star. Disk fragmentation into sub-stellar objects could occur in the cold exterior part of the disk. Direct detection of massive gaseous clumps on their way to collapse into gas giant planets would offer an unprecedented test of the disk instability model. Here we use state-of-the-art 3D radiation-hydro simulations of disks undergoing fragmentation into massive gas giants, post-processed with the RADMC-3D ray-tracing code to produce dust continuum emission maps. These are then fed into the Common Astronomy Software Applications (CASA) ALMA simulator. The synthetic maps show that both overdense spiral arms and actual clumps at different stages of collapse can be detected with the Atacama Large Millimetre/sub-millimetre Array (ALMA) in the full configuration at the distance of the Ophiuchus star forming region (125 pc). The detection of clumps is particularly effective at shorter wavelengths (690 GHz) combining two resolutions with multi-scale clean. Furthermore, we show that a flux-based estimate of the mass of a protoplanetary clump can be from comparable to a factor of 3 higher than the gravitationally bound clump mass. The estimated mass depends on the assumed opacity, and on the gas temperature, which should be set using the input of radiation-hydro simulations. We conclude that ALMA has the capability to detect "smoking gun" systems that are a signpost of the disk instability model for gas giant planet formation.