2026MNRAS.545f2056K

Our knowledge of the chemical composition of the gas in the inner disc of intermediate-mass young stars is limited, due to the lack of suitable instrumentation. The launch of James Webb Space Telescope (JWST) has provided a significant improvement in our ability to probe gas in these inner discs. We analyse the gas composition and emitting conditions of the disc around HD 35929, a young intermediate-mass Herbig star, using data from the Mid-Infrared Instrument (MIRI) in the medium-resolution mode (MRS). Our goal is to constrain the chemistry and kinematics of the gas phase molecules detected in the inner disc. We use the interactive Spectral-Line Analysis Tool (iSLAT) to examine the observed molecular lines and the Dust Continuum Kit with Line Emission from Gas (DuCKLiNG) to detect, fit, and analyse the molecular emission. We find gas phase H$_2$O, CO, CO$_2$, and OH in the disc, as well as H I recombination lines. Surprisingly, we also detect gas phase SiO in the fundamental v = 1-0 vibrational band. We derive column densities and temperature ranges of the detected species, arising from the inner ∼0.2 au, hinting towards a compact and very warm disc. The molecular column densities are much higher than found in lower mass T Tauri discs. In general, the molecular composition is consistent with an O-rich gas from which silicate-rich solids condense and the strong gas phase molecular line emission suggests a low dust opacity. The unexpected detection of gas phase SiO at the source velocity points to an incomplete condensation of rock forming elements in the disc, suggesting chemical disequilibrium and/or an underestimate of the gas kinetic temperature.