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

Problem and Motivation
Binary and multiple star systems constitute the majority of stellar populations, where dynamical interactions significantly influence disc evolution and planet formation potential. While facilities like ALMA and the VLT have confirmed dynamical effects such as disc truncation, infrared observations of these systems have historically been limited by spatial resolution. Previous surveys using the Spitzer Space Telescope could not resolve individual components in binary systems, resulting in spectra dominated by the primary star. The advent of the James Webb Space Telescope (JWST) and its Mid-Infrared Instrument (MIRI) offers the necessary spatial resolution and sensitivity to disentangle the spectra of binary components in the mid-infrared (4.9–27.9 µm). However, early JWST/MIRI observations of binaries (e.g., DF Tau, AS 205, VW Cha) revealed a trend where primary stars exhibit rich molecular emission while secondaries are line-poor, a phenomenon potentially driven by disc truncation and inclination effects. Furthermore, while ice spectroscopy is a key focus of infrared astronomy, observing ice absorption features in Class II discs requires edge-on configurations (>75° inclination) to block the central star's continuum, a condition rarely met in resolved binary studies.

Methodology
This study presents JWST/MIRI Medium Resolution Spectrometer (MRS) observations of the pre-main sequence binary system HK Tau (HK Tau A and B), taken as part of the MINDS (MIRI Mid-Infrared Disk Survey) GTO program. The system is located in the Taurus star-forming region at a distance of 128 pc and consists of two roughly equal-mass M-type stars. Crucially, the system exhibits complementary inclinations: HK Tau A is viewed at a low inclination (57°), while HK Tau B is nearly edge-on (83°).

The data were reduced using the MINDS pipeline, a hybrid of the standard JWST pipeline and the VIP package. To separate the spectra of the two sources, which are separated by 2.4 arcseconds but suffer from overlapping Point Spread Function (PSF) wings at longer wavelengths, the authors employed a forward modeling approach combined with aperture photometry. This method utilizes theoretical PSFs for each channel to subtract the companion's contribution, ensuring the extracted spectra are not contaminated by the PSF wings of the other source.

The analysis involved:

1. Spectral Characterization: Identifying molecular emission lines (H2O, CO2, HCN, OH) and atomic forbidden lines ([Ar II], [Ne II], [Ne III]) in the continuum-subtracted spectra.
2. Modeling: Fitting Local Thermodynamic Equilibrium (LTE) 0D slab models to the molecular emission of HK Tau A to derive temperature and column density.
3. Ice Analysis: Analyzing absorption bands in the HK Tau B spectrum to identify solid-phase species (H2O, CO2, NH4+, CH4, silicates).
4. Extended Emission Mapping: Creating moment maps for extended H2 emission and performing a pixel-by-pixel rotational diagram analysis to derive 2D maps of gas temperature and column density.
5. Radiative Transfer Comparison: Comparing observed trends with synthetic spectra generated from a fiducial T Tauri disc model (using ProDiMo and FLiTs) to assess the impact of inclination on molecular visibility.

Key Results

• Complementary Spectral Features: HK Tau A displays a line-rich spectrum dominated by CO2 and H2O emission, consistent with a "CO2-dominated" source. In contrast, HK Tau B is strikingly line-poor in molecular emission, showing only atomic ion lines ([Ar II], [Ne II], [Ne III]) and extended H2.
• Inclination Effects: Radiative transfer modeling confirms that at inclinations >75°, molecular emission from the disc midplane is obscured, leaving only ionized atomic lines from the upper disc layers or winds visible. This explains the lack of molecular lines in HK Tau B.
• Ice Detection: The edge-on configuration of HK Tau B allows for the detection of ice absorption bands against the continuum. The authors confidently detect H2O ice (6.2 and 13.6 µm), CO2 ice (15.2 µm), and NH4+ ice (6.85 µm). A broad absorption between 8.3 and 9 µm is compatible with silicate stretching and C-H bending. No Polycyclic Aromatic Hydrocarbon (PAH) emission is detected in either source.
• Extended H2 and Winds: Extended H2 emission is detected around both stars but is significantly more elongated around HK Tau B, forming a distinctive 'X' shape. The semi-opening angle of this structure is measured at ~70°–77°. The morphology, combined with the hourglass shape of the [Ne II] emission and the temperature/density structure derived from the rotational diagram, suggests a wind origin (either photoevaporative or magnetohydrodynamic) rather than a collimated jet.
• Ionization and Irradiation: The detection of [Ne III] in HK Tau B and the line ratios ([Ne III]/[Ne II] and [Ne II]/[Ar II]) indicate that the disc ionization is dominated by X-ray irradiation. The presence of [Ne II] and [Ar II] in HK Tau A, but the absence of [Ne III], suggests that any wind from the primary is too cold or dense to be ionized to the degree required for [Ne III] emission.

Significance and Claims
The paper claims that the HK Tau system provides an unprecedented opportunity to simultaneously view the gaseous and solid components of a pair of coeval protoplanetary discs. The complementary inclinations allow researchers to observe the molecular gas reservoir in the primary (HK Tau A) and the ice reservoir in the secondary (HK Tau B) within the same system, minimizing evolutionary differences.

The authors conclude that the dichotomy between the line-rich primary and line-poor secondary is fundamentally driven by the viewing geometry (inclination) rather than solely by evolutionary truncation, as the mass ratio is near unity. The study highlights that sources at the interface between face-on and edge-on configurations (>75°) are uniquely positioned to reveal both gas-phase and ice-phase features. Additionally, the lack of PAH emission in HK Tau B, contrasting with other edge-on discs observed by MIRI, suggests that PAH detection in edge-on systems may depend on specific geometric or irradiation conditions that require further statistical investigation. This work represents a step forward in characterizing multiple systems and edge-on discs with JWST, emphasizing the need for larger samples to interpret these findings within a broader population framework.