MINDS: The very low-mass star and brown dwarf sample

II. Probing disk settling, dust properties, and dust-gas interplay with JWST/MIRI

¹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
² Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, Postbus 800, 9700AV Groningen, The Netherlands
³ Department of Physics and Astronomy, University of Exeter, Exeter EX4 4QL, UK
⁴ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁵ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁶ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁸ Max-Planck Institut für Extraterrestrische Physik (MPE), Giessen-bachstr. 1, 85748 Garching, Germany
⁹ Niels Bohr Institute, University of Copenhagen, NBB BA2, Jagtvej 155A, 2200 Copenhagen, Denmark
¹⁰ Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA
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 Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁴ Earth and Planets Laboratory, Carnegie Institution for Science, 5241 Broad Branch Road, NW, Washington, DC 20015, USA
¹⁵ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
¹⁶ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium

^★ Corresponding author: hyerin.jang@astro.ru.nl

Received: 1 July 2025
Accepted: 27 August 2025

Abstract

Context. Disks around very low-mass stars (VLMSs) provide environments for the formation of Earth-like planets. Mid-infrared observations have revealed that these disks often exhibit weak silicate features and prominent hydrocarbon emissions.

Aims. This study aims to characterize the dust properties and geometrical structures of VLMS and brown dwarf (BD) disks, observed by the James Webb Space Telescope (JWST)/Mid-Infrared Instrument (MIRI). We investigate how these properties relate to gas column density and potential evolutionary stages.

Methods. We analyzed mid-infrared spectra of ten VLMSs and BD disks from the JWST/MIRI observations as part of the MIRI mid-Infrared Disk Survey (MINDS) program. Spectral slopes and silicate band strengths were measured and compared with hydrocarbon emission line ratios, which probe the gas column density. Moreover, the Dust Continuum Kit with Line emission from Gas (DuCKLinG) was used to quantify grain sizes, dust compositions, and crystallinity on the disk surface.

Results. The disks are classified into less, more, and fully settled geometries based on their mid-infrared spectral slopes and silicate band strengths. Less settled disks show a relatively strong silicate band, high spectral slopes, and low crystallinity and are dominated by 5 µm-sized grains. More settled disks have weaker silicate bands, low spectral slopes, enhanced crystallinity, and higher mass fractions of smaller grains (<5 µm). Fully settled disks exhibit little or no silicate emission and negative spectral slopes. An overall trend of increasing gas column density with decreasing spectral slope suggests that more molecular gas is exposed when the dust opacity decreases with increasing dust settling.

Conclusions. Our findings indicate that our sample shows dust processing signatures of grain growth and crystallization. These characteristics may reflect possible evolutionary pathways with disk turbulence, dust settling, and thermal processing or may alternatively point to inner-disk clearing or a collisional cascade. These results highlight the need for broader samples to understand the link between dust and gas appearance in regions where Earth-like planets form.