MINDS: A transition from H₂O to C₂H₂ dominated disk spectra with decreasing stellar luminosity

¹ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
² Earth and Planets Laboratory, Carnegie Institution for Science, 5241 Broad Branch Road, NW, Washington, DC 20015, USA
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁵ Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, Postbus 800, 9700AV Groningen, The Netherlands
⁶ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
⁷ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁸ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁹ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, D02 XF86 Dublin, Ireland
¹⁰ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
¹¹ Dept. of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
¹² ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland
¹³ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁴ Department of Physics and Astronomy, University of Exeter, Exeter EX4 4QL, UK
¹⁵ Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁶ Niels Bohr Institute, University of Copenhagen, NBB BA2, Jagtvej 155A, 2200 Copenhagen, Denmark
¹⁷ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands

^★ Corresponding author: sgrant@carnegiescience.edu

Received: 7 June 2025
Accepted: 5 August 2025

Abstract

Context. The chemical composition of the inner regions of disks around young stars will largely determine the properties of planets that form in these regions. Many physical processes in the disks drive their chemical evolution, and some of them depend on and/or correlate with the stellar properties.

Aims. We explore the connection between stellar properties and the chemistry of the inner disk in protoplanetary disks as traced by mid-infrared spectroscopy.

Methods. We used JWST-MIRI observations of a large diverse sample of sources to explore trends between the carbon-bearing molecule C₂H₂ and the oxygen-bearing molecule H₂O. Additionally, we calculated the average spectrum for the T Tauri (M_*>0.2 M_⊙) and very low-mass star (VLMS; M_*,≤0.2 M_⊙) samples from JWST-MIRI MRS data and used slab models to determine the properties of the average spectra in each subsample.

Results. We find a significant anticorrelation between the flux ratio of C₂H₂/H₂O and the stellar luminosity. The F_C2H2/F_H2O flux ratios of disks around VLMSs are significantly higher than the fluxes in their higher-mass counterparts. This is driven by the generally weak H₂O and strong C₂H₂ in disks around low-mass hosts. We also explored trends with the strength of the 10 µm silicate feature, the stellar accretion rate, and the disk dust mass. They are all correlated with F_C2H2/F_H2O, which may be related to processes that drive the carbon enrichment in disks around VLMSs, but are also degenerate with the system properties (i.e., the M_*−Ṁ and M_*−M_disk relations). Slab model fits to the average spectra show that H₂O emission in the VLMS sample is quite similar in temperature and column density to a warm (~600 K) H₂O component in the T Tauri spectrum. This indicates that the high C/O gas-phase ratio in these disks is not due to oxygen depletion alone. Instead, the many hydrocarbons, including some with high column densities, suggest that carbon enhancement occurs in the disks around VLMSs.

Conclusions. The observed differences in the chemistry of the inner disk as a function of host properties are likely to be accounted for by differences in the disk temperatures, stellar radiation field, and the evolution of dust grains.