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

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 CO 2 -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, CO 2 ice at 15.2 μm, and NH 4 + 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 H 2 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 H 2 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.