The circumbinary disk of HD34700A

I. CO gas kinematics indicate spirals, infall, and vortex motions

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
³ Max-Planck Institute for Astronomy (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁴ Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
⁵ NASA Hubble Fellowship Program Sagan Fellow
⁶ Department of Physics & Astronomy, University of Victoria, Victoria, BC V8P 5C2, Canada
⁷ Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁸ Heising-Simons Foundation 51 Pegasi b Fellow
⁹ Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
¹⁰ Universitá degli Studi di Milano, via Celoria 16, 20133 Milano, Italy
¹¹ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 29 July 2025
Accepted: 16 January 2026

Abstract

Aims. We present the first high-resolution (∼0″.14) Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 dust continuum, ¹²CO J = 2-1, ¹³CO J = 2-1, and C¹⁸O J = 2-1 molecular line emission observations of the quadruple system HD 34700. In particular, HD34700AaAb is a spectroscopic binary (M_bin = 4 M_⊙) surrounded by two low-mass companions (B = 0.6 M_⊙, C = 0.4 M_⊙) at large separations. Its circumbinary disk is highly substructured, featuring numerous spiral arms and a large cavity observed in infrared (IR) scattered light. We aim to shed light on the nature of these features by examining the gas kinematics at work in the circumbinary disk.

Methods. We analyzed the CO line channel and intensity moment maps. By fitting a Keplerian model to the line channel emission, we identified the residual motions and conducted a line spectra analysis.

Results. We resolved an asymmetric continuum crescent on top of a dust ring at 0739 (138 au) colocated with the IR ring. The CO molecule’s line emission traces a smaller cavity in gas, whose edge aligns with the inner rim of the ring detected in Hα emission at 0″.20 (65 au). The ¹²CO line emission and kinematics trace highly non-Keplerian motions (∼0.1Δυ_k) and these CO spiral features align well with the spiral structures in scattered light. The ¹²CO line spectra analysis reveals a streamer above the southeastern disk plane, likely falling onto the disk. The ¹³CO and C¹⁸O kinematics largely follow the disk’s underlying Keplerian rotation, while ¹³CO exhibits tentative signs of anticyclonic vortex flows at the continuum crescent location.

Conclusions. Our multimolecular line study suggests that the circumbinary disk of HD 34700A is highly perturbed in its upper layers, possibly warped and influenced by infalling material. While late-stage infall may account for the IR spirals and the formation of the vortex through Rossby wave instability, an embedded massive companion within the cavity might also be contributing to these features.