An interferometric mid-infrared study of the eruptive star binary Z CMa with MATISSE/VLTI

I. Imaging the protoplanetary disks during the 2023 outburst^★

¹ Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
² MTA Centre of Excellence, HUN-REN CSFK, Budapest, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
³ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, UT3-PS, OMP, CNRS, 9 av. du Colonel Roche, 31028 Toulouse Cedex 4, France
⁴ Institute of Physics and Astronomy, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
⁵ Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁶ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁷ Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Boulevard de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
⁸ Institut für Theoretische Physik und Astrophysik, Christian-Albrechts-Universität zu Kiel, Leibnizstraße 15, 24118 Kiel, Germany
⁹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁰ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹¹ Planetarium im Museum am Schölerberg, Klaus-Strick-Weg 10, 49082 Osnabrück, Germany
¹² NASA Goddard Space Flight Center, Astrophysics Division, Greenbelt, MD 20771, USA
¹³ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France

^★★ Corresponding author: foteini.lykou@csfk.org

Received: 10 December 2024
Accepted: 13 August 2025

Abstract

Aims. The current work is part of a series aimed at producing the first ever mid-infrared images of protoplanetary disks in the binary system of eruptive stars Z CMa and studying their individual properties.

Methods. We obtained high-angular-resolution interferometric observations with MATISSE/VLTI in the L (2.9–4.1 μm), M (4.5–4.9 μm), and N (8–13 μm) bands, as well as spectroscopic observations in the near-infrared (NIR) with SpeX/IRTF. We present our quantitative analysis on the interferometric data using geometric model fitting, image reconstruction algorithms, and orbital simulation tools, and we compare our findings to those of literature studies.

Results. The mid-infrared (MIR) emitting regions of the individual protoplanetary disks in the binary system Z CMa are resolved by MATISSE/VLTI. The observations were obtained during a serendipitous large outburst of the Herbig (HBe) star that lasted more than 100 days, while the FU Orionis-type (FUor) companion is presumed to be in quiescence. The size of the MIR-emitting disk region of the more massive HBe star increases toward longer wavelengths from <14 mas at 3.5 μm to ≪50 mas at 11.5 μm. The lack of substructures in the HBe disk might suggest that it is a continuous disk; however, this could be due to observational constraints. We also note a radial variation of the silicate absorption feature over the disk, where the optical depth increases inwards of <40 au radii. This contradicts the scenario of a carved, dusty cocoon surrounding the HBe star. In the case of the less massive FUor companion, the MIR-emitting region is much smaller with an angular size ≤15 mas (or else a physical radius <9 au) in all bands, suggesting a compact disk. Both disks are aligned within uncertainties, and their orientation agrees with that of the known jets. Furthermore, MATISSE data place the binary’s separation at 117.88 ± 0.73 mas and a position angle of 139.16° ± 0.29° east of north. Our estimates for the orbital elements gave an eccentric orbit (e ~ 0.17) with a moderate inclination (i ~ 66°). The derived total mass is M_total = 16.4_−2.3^+2.1 M_⊙, while the period is approximately 950 years.

Conclusions. Our MATISSE imaging of the Herbig disk during outburst indicates a temperature gradient for the disk, while imaging of the FUor companion’s disk corroborates previous studies showing that FUor disks are rather compact in the MIR. We cannot infer any misalignment between the MATISSE results and earlier ALMA/JVLA data, nor can we infer any influence from the alleged flyby event.