Letter to the Editor

Direct imaging of mass transfer and circumcompanion structures in π¹ Gru with VLTI/MATISSE

¹ European Southern Observatory Alonso de Córdova 3107 Vitacura Santiago, Chile
² Division of Astronomy and Space Physics, Uppsala University Uppsala, Sweden
³ Department of Space, Earth and Environment, Chalmers University of Technology Göteborg, Sweden
⁴ Institut d’Astronomie et d’Astrophysique, Université libre de Bruxelles Bruxelles, Belgium
⁵ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS Paris, France
⁶ Centro de Astrobiologia (CAB), CSIC-INTA Madrid, Spain
⁷ Institute of Fundamental Physics (CSIC), Department of Molecular Astrophysics Madrid, Spain
⁸ Department of Astrophysics, University of Vienna Vienna, Austria
⁹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange Nice, France
¹⁰ Instituto de Astrofísica, Universidad Andrés Bello Santiago, Chile
¹¹ Max Planck Institute for Radio Astronomy Bonn, Germany
¹² European Southern Observatory Garching, Germany
¹³ Center for High Angular Resolution Astronomy, Georgia State University California, USA
¹⁴ Max-Planck-Institut für Astrophysik Garching, Germany
¹⁵ Schmidt Sciences New York, USA
¹⁶ HUN-REN Konkoly Observatory Budapest, Hungary
¹⁷ National Science Foundation Alexandria, USA
¹⁸ Universidad Nacional Autónoma de México, Instituto de Astronomía México City, Mexico

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

Received: 28 November 2025
Accepted: 4 January 2026

Abstract

Aims. We investigate how the presence of a binary companion appears to affect dust and molecule formation in the circumstellar environment of a star on the asymptotic giant branch (AGB).

Methods.L- and N-band observations obtained over the course of one month with the multi aperTure mid-infrared spectroScopic experiment instrument (MATISSE) at the very large telescope interferometer (VLTI) were used to constrain the distribution of dust and molecules in the close environment of π¹ Gru through image reconstruction. The reconstructed images were fit to the interferometric observables, that is, the visibilities and closure phases, using the two Python-based tools Python for MiRA (PYRA) and mean astrophysical images with PYRA (MYTHRA) built around the multi-aperture image reconstruction algorithm (MiRA).

Results. Our observations support (i) a wind Roche-lobe overflow (WRLOF) scenario, where mass transfer from the AGB star to its companion produces a possible circumcompanion disk in the L band around π¹ Gru C, with a central cavity that likely traces dust sublimation and a circumcompanion envelope in the N band. (ii) A main-sequence nature for the companion: Because both Atacama large millimeter/submillimeter array (ALMA) and the VLTI/MATISSE N-band observations show emission, we favor a thermal infrared emission from the main-sequence star over free-free emission from a with dwarf companion. Finally, (iii) a plume-like structure extending from π¹ Gru C, likely marking the onset of the spiral observed at larger scales. Together, this provides direct evidence that links small-scale mass transfer and disk formation through WRLOF to the global circumstellar morphology.

Conclusions. These results highlight the atmospheric deformation induced by the companion, reveal ongoing mass transfer between the evolved star and its companion, and indicate a circumcompanion disk-like structure.