MINDS: The molecule-rich disc of the Herbig star HD 35929 revealed with JWST/MIRI

TL;DR

This work uses JWST/MIRI MRS data to probe the inner disc gas of the intermediate-mass Herbig star HD 35929, revealing a rich, warm molecular inventory and constraining the gas kinematics. By applying iSLAT for line identification and DuCKLinG for Bayesian slab modelling, the authors detect H$_2$O, CO, CO$_2$, OH, SiO, and HI, with water and SiO showing particularly high column densities and compact emitting regions near the star ($\sim$0.1–0.2 au). The analysis demonstrates that the inner disc of HD 35929 is unusually molecule-rich for a Herbig system, with SiO indicating non-equilibrium chemistry and/or underestimated gas temperatures. While the modelling provides robust detections and constraints, it remains limited by LTE slab assumptions; future full thermochemical disc modelling is needed to translate column densities into volume densities and to capture radiative transfer effects. Overall, the study highlights a compact, gas-rich inner disc around a luminous Herbig star, offering new insights into the chemistry and evolution of inner discs in intermediate-mass systems.

Abstract

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 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 MIRI/MRS data. Our goal is to constrain the chemistry and kinematics of the gas phase molecules detected in the inner disc. We use iSLAT to examine the observed molecular lines and DuCKLiNG to detect, fit, and analyse the molecular emission. We find gas phase H2O, CO, CO2, and OH in the disc, as well as HI 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.

Figures (13)

• Figure 1: Spectral Energy Distribution of HD 35929. The blue curve is a Kurucz model with T$_{\rm eff}$ = 6500 K and log g = 3.5, reddened with A$_V$ = 0.20 mag. The orange and grey lines represent the JWST/MIRI and Spitzer/IRS spectra, respectively. Also shown are photometric points (Strömgren, GAIA, 2MASS, IRAC, WISE, and IRAS).
• Figure 2: JWST/MIRI MRS spectrum (orange) of HD 35929. The Spitzer/IRS spectrum (grey) is shown for comparison. The insets highlight the emission from CO, CO2, H2O, and most notably the SiO fundamental vibrational mode. The vertical lines in the SiO panel indicate the positions of the rotational lines with upper level energies lower than 3600 K.
• Figure 3: Full Width at Half Maximum line widths of selected emission lines as a function of their Doppler shift. A stellar radial velocity of 22 km s$^{-1}$ was adopted.
• Figure 4: Continuum subtracted MIRI spectrum, using the 4.9 - 25 $\mu$m wavelength range, but excluding the 10 $\mu$m silicate band region between 9.0 and 13.5 $\mu$m (excluded region marked in grey). Overplotted are the cumulative fluxes from H$_2$O, CO, SiO, CO$_2$, and OH from the median probability model. A few unfitted atomic lines and the non-detection of ^13CO2 are labelled. The apparent feature around $18.55\,\rm \mu m$ can be traced back to a bad pixel artifact.
• Figure 5: Flux from the base model (blue) and a model including C2H2 (red) compared to the observation (black) in the region where C2H2 emission is expected. The lines indicate the median posterior flux, with the shaded region indicating the $1\sigma$ level.