# On the ALMA observability of nascent massive multiple systems formed by gravitational instability

## TL;DR

This study uses radiative transfer calculations and synthetic ALMA images to demonstrate that substructures like gaseous clumps and spiral arms in massive young stellar objects' accretion discs are observable with current ALMA capabilities, revealing insights into disc fragmentation.

## Contribution

The paper provides the first detailed simulation-based analysis of ALMA's ability to detect disc substructures in forming massive stars, highlighting the impact of gravitational instability on observability.

## Key findings

- ALMA can resolve disc substructures at 1.2 mm within 1-2 kpc.
- Substructures are observable regardless of viewing angle.
- Disc evolution increases the likelihood of detecting clumps.

## Abstract

Massive young stellar object (MYSOs) form during the collapse of high-mass pre-stellar cores, where infalling molecular material is accreted through a centrifugally-balanced accretion disc that is subject to efficient gravitational instabilities. In the resulting fragmented accretion disc of the MYSO, gaseous clumps and low-mass stellar companions can form, which will influence the future evolution of massive protostars in the Hertzsprung-Russell diagram. We perform dust continuum radiative transfer calculations and compute synthetic images of disc structures modelled by the gravito-radiation-hydrodynamics simulation of a forming MYSO, in order to investigate the Atacama Large Millimeter/submillimeter Array (ALMA) observability of circumstellar gaseous clumps and forming multiple systems. Both spiral arms and gaseous clumps located at ~a few 100 au from the protostar can be resolved by interferometric ALMA Cycle 7 C43-8 and C43-10 observations at band 6 (1.2 mm), using a maximal 0.015" beam angular resolution and at least 10-30 min exposure time for sources at distances of 1-2 kpc. Our study shows that substructures are observable regardless of their viewing geometry or can be inferred in the case of an edge-viewed disc. The observation probability of the clumps increases with the gradually increasing efficiency of gravitational instability at work as the disc evolves. As a consequence, large discs around MYSOs close to the zero-age-main-sequence line exhibit more substructures than at the end of the gravitational collapse. Our results motivate further observational campaigns devoted to the close surroundings of the massive protostars S255IR-NIRS3 and NGC 6334I-MM1, whose recent outbursts are a probable signature of disc fragmentation and accretion variability.