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

TL;DR

This work analyzes JWST/MIRI observations of ten very low-mass star and brown dwarf disks to investigate dust settling, grain sizes, and crystallinity, and their connection to gas density in the inner disk. Using the DuCKLinG retrieval framework, the authors decompose mid-IR spectra into dust and gas components across a five-species dust suite and multiple grain sizes, revealing a progression from less-settled disks dominated by ~5 μm grains to more- and fully-settled disks with higher crystallinity and smaller-grain fractions, and weak silicate emission in the most evolved cases. They find an overall trend where gas column density, traced by hydrocarbon ratios such as $F_{^{13} m CCH_2}/F_{ m C_{2}H_{2}}$, increases as the dust opacity decreases, consistent with deeper optical-depth probing in settled disks, and they document a nuanced crystallinity pattern in which enstatite becomes prominent in inner regions while forsterite remains outer-disk dominated. The results point to evolutionary pathways involving dust settling, thermal processing, inner-disk clearing, or collisional cascades, and underscore the need for broader VLMS disk samples to link dust and gas appearance to planet-forming environments around VLMS and BD hosts.

Abstract

Disks around very low-mass stars (VLMS) provide environments for the formation of Earth-like planets. Mid-infrared observations have revealed that these disks exhibit weak silicate features and strong hydrocarbon emissions. This study characterizes 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), and connects these to gas column density and potential evolutionary stages. We analyze mid-infrared spectra of ten VLMS and BD disks as a part of the MIRI mid-Infrared Disk Survey (MINDS) program. Spectral slopes and silicate band strengths are compared with hydrocarbon emission line ratios, which probe the gas column density. Moreover, the Dust Continuum Kit with Line emission from Gas is used to quantify grain sizes, dust compositions, and crystallinity in the disk surface. 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 $μ$m-sized grains. More-settled disks have weaker silicate band, low spectral slope, enhanced crystallinity, and higher mass fractions of smaller grains. 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 due to dust settling. Our findings may reflect possible evolutionary pathways with dust settling and thermal processing or may 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.

Figures (14)

• Figure 1: Spectra of VLMS disks in MINDS sample observed by JWST/MIRI MRS. The gray line is the MIRI spectrum, the black line is the rebinned spectrum to $R\sim200$. For NC9, HKCha, IC147, the green line is the foreground extinction corrected spectrum. The spectra are ordered based on their spectral slopes within the mid-IR range.
• Figure 2: Strengths and shapes of the 10 $\mu$m silicate bands. The VLMS disks are shown in black dots, and gray dots are T Tauri disks observed with Spitzer IRS extracted from the CASSIS database Lebouteiller_etal2011. The small panel zooms into the region, where the VLMS disks are located.
• Figure 3: Strength of the 10 $\mu$m silicate band and $12.6-22.5~\mu$m spectral slope of the VLMS disks. The silicate bands for J1605, TWA27, IC147 are set to be unity due to the absence of a clear silicate band. The F9.8 value of IC147 from its decomposed disk components in the DuCKLinG model is F9.8=1.23.
• Figure 4: Gas column density and spectral slope. $F_{^{13}\rm CCH_{2}}/F_{\rm C_{2}H_{2}}$ represents gas column density, measured in Paper I. J1558 is the upper limit value for $F_{^{13}\rm CCH_{2}}/F_{\rm C_{2}H_{2}}$.
• Figure 5: DuCKLinG fitting results for NC1, J1558, NC9, J0439, HKCha, and Sz28. The final model is the black solid line, and the rebinned MIRI spectrum is the orange line. The gray dashed, dotted, and dash-dotted lines are inner rim and stellar (red line) combined component, the midplane component, and the disk surface component, respectively. The blue line is the gas emission component.