MINDS: The very low-mass star and brown dwarf sample: II. Probing disk settling, dust properties, and dust-gas interplay with JWST/MIRI

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.