Spiral arms and instability within the AFGL 4176 mm1 disc

TL;DR

This study uses high-resolution ALMA observations to analyze the structure and stability of the disc around the forming O-type star AFGL 4176 mm1, revealing spiral arms and signs of gravitational instability leading to potential fragmentation.

Contribution

It provides detailed modeling of the spiral structure and assesses the gravitational stability of the disc, highlighting the role of fragmentation in high-mass star formation.

Findings

The disc contains a spiral arm best modeled by an Archimedean spiral.

Outer disc regions are gravitationally unstable and likely to fragment.

Evidence supports disc fragmentation as a pathway to multiple star systems.

Abstract

We present high-resolution (30 mas or 130 au at 4.2 kpc) Atacama Large Millimeter/submillimeter Array observations at 1.2 mm of the disc around the forming O-type star AFGL 4176 mm1. The disc (AFGL 4176 mm1-main) has a radius of ~1000 au and contains significant structure, most notably a spiral arm on its redshifted side. We fitted the observed spiral with logarithmic and Archimedean spiral models. We find that both models can describe its structure, but the Archimedean spiral with a varying pitch angle fits its morphology marginally better. As well as signatures of rotation across the disc, we observe gas arcs in CH$_3$CN that connect to other millimetre continuum sources in the field, supporting the picture of interactions within a small cluster around AFGL 4176 mm1-main. Using local thermodynamic equilibrium modelling of the CH$_3$CN K-ladder, we determine the temperature and velocity field across the disc, and thus produce a map of the Toomre stability parameter. Our results indicate that the outer disc is gravitationally unstable and has already fragmented or is likely to fragment in the future, possibly producing further companions. These observations provide evidence that disc fragmentation is one possible pathway towards explaining the high fraction of multiple systems around high-mass stars.