MINDS: Complementary inclinations in the binary system HK Tau reveal gas- and ice-phase chemistry

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
² Niels Bohr Institute, University of Copenhagen, NBB BA2, Jagtvej 155A, 2200 Copenhagen, Denmark
³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, Postbus 800, 9700AV Groningen, The Netherlands
⁶ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁷ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁸ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
⁹ Earth and Planets Laboratory, Carnegie Institution for Science, 5241 Broad Branch Road, NW, Washington, DC 20015, USA
¹⁰ Department of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
¹¹ ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland
¹² Department of Physics and Astronomy, University of Exeter, Exeter EX4 4QL, UK
¹³ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France

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Received: 2 March 2026
Accepted: 16 June 2026

Abstract

HK Tau is a roughly equal-mass pre-main-sequence binary system consisting of a low-inclination primary (57°) and an edge-on (83°) secondary. We present JWST Mid-Infrared Instrument (MIRI) observations targeting both sources, taken as part of the JWST GTO program MIND. The mid-infrared spectra reveal a line-rich CO₂-dominated primary and a line-poor secondary. This evidence, albeit in line with the evolution-motivated trend uncovered by recent observations of binaries at MIRI wavelengths, is likely due to the different configuration of the two sources. Thermochemical disc models coupled with radiative transfer indeed show that at inclinations comparable to that of HK Tau B, only ionised atomic lines are expected to remain visible in the spectra. While it blocks molecular emission lines, however, the edge-on configuration allows ice absorption bands to be visible against the continuum. In this framework, the HK Tau system provides an unprecedented opportunity for a simultaneous view of the solid and gaseous component of a pair of coeval protoplanetary discs thanks to the complementary inclination of the two sources. We detect water ice at 6.2 and 13.6 μm, CO₂ ice at 15.2 μm, and NH₄⁺ ice at 6.85 μm in the spectrum of HK Tau B. An additional absorption band between 8.3 and 9 μm is compatible with both silicate stretching and C-H bending. Neither the primary nor the secondary shows signs of polycyclic aromatic hydrocarbons. Extended H₂ emission is present around both sources, but is much more elongated in HK Tau B. The distinctive X shape centred at the location of B, combined with the intensity, morphology, and spectral characteristics of the ionised atomic lines [Ar II], [Ne II], and [Ne III], suggests a low-velocity wind origin with a wide (~ 70°) semi-opening angle. The lower forbidden line fluxes and smaller spatial extent of the H₂ emission around A imply that if a wind is launched from the primary as well, it is too cold or dense to be ionised and emit brightly.