Letter to the Editor

Spiral arms and instability within the AFGL 4176 mm1 disc

¹ School of Physics & Astronomy, E.C. Stoner Building, The University of Leeds, Leeds LS2 9JT, UK
e-mail: k.g.johnston@leeds.ac.uk
² Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
³ Institute of Astronomy and Astrophysics, Eberhard Karls University Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
⁴ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁵ Moscow Institute of Physics and Technology, 9 Institutskiy Per., Dolgoprudny 141701, Russia
⁶ Ural Federal University, 19 Mira St., Ekaterinburg 620075, Russia
⁷ ESO Headquarters, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁸ Aperio Software, Headingley Enterprise and Arts Centre, Bennett Road, Headingley, Leeds LS6 3HN, UK

Received: 20 November 2019
Accepted: 21 January 2020

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 CH₃CN 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 CH₃CN 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.