The edge-on disc Tau 042021: Icy grains at high altitudes and a wind containing astronomical polycyclic aromatic hydrocarbons

Context. Spectra of the nearly edge-on protoplanetary discs observed with the JWST have shown ice absorption bands of varying optical depths and peculiar profiles, challenging radiative transfer modelling and our understanding of dust and ice in discs. Aims. With the aim of constraining the underlying disc’s structure and evolutionary state, we build models including dust grain size, shape, and composition to reproduce JWST IFU spectroscopy of a well-characterised, massive, and large edge-on disc, Tau 042021. Specifically, we aim to match its spectral energy distribution, the spatial distribution of the dust and ice, and the spectral characteristics of the dust continuum and ice bands profiles, as well as test for the presence of astronomical polycyclic aromatic hydrocarbons (PAH) band carriers. Methods. We explored radiative transfer models using different dust grain size distributions, including grains with effective radii of aeff = 0.005–3000 μm. Mass absorption and scattering coefficients for distributions of triaxial ellipsoidal grains were calculated using the discrete dipole approximation (DDA) for small size parameters (2πaeff/λ < 10), whereas the hollow sphere approximation was used for larger size parameters. We considered compositions including silicates, amorphous carbon, and mixtures of water, carbon dioxide, and carbon monoxide. The resulting orientation-averaged scattering matrices were input into RADMC-3D Monte Carlo radiative transfer models of Tau 042021 to simulate the spectral cubes observed with JWST-NIRSpec and MIRI. We compared the calculated optical depth distributions and profiles of the main ice bands to observations, including water at 3.05 μm, carbon monoxide at 4.67 μm, and carbon dioxide at 4.26 μm. We also compared these results to archival JWST-NIRCam and ALMA continuum images. We tested three increasingly complex disc structures, starting from a standard model and adding first an extended atmosphere, then a disc wind containing astro-PAHs. Results. The observed near- to mid-infrared spectral energy distribution requires efficient scatterers, implying dust distributions that include grains of up to several tens of microns in size. The intensity distribution perpendicular to the disc exhibits emission profile wings extending into the upper disc atmosphere at altitudes exceeding the classical scale height expected in the isothermal hydrostatic limit. We produce ice absorption images that demonstrate the presence of icy dust grains up to altitudes high above the disc midplane, more than three hydrostatic equilibrium scale heights. We demonstrate the presence of a wind containing the carriers of astronomical PAH bands. The wind appears as an X-shaped emission at 3.3, 6.2, 7.7, and 11.3 μm, characteristic wavelengths associated with the infrared astronomical PAH bands. We associate the spatial distribution of this component with carriers of astronomical PAH bands that form a layer of emission at the interface with the H2 wind.