Hyerin Jang

Context . The JWST gives unique access to the chemical and physical conditions in inner disks ( 0.4 M _⊙ ). In V1094 Sco, even cold C ₄ H ₂ is detected. In contrast, the IM Lup spectrum is dominated by O-bearing species. No one-to-one correspondence is found between the gas in the outer disk, as traced by the C ₂ H (3–2)/C ¹⁸ O (2–1) flux ratios, and that of the inner disk, as traced by the C ₂ H ₂ /H ₂ O flux ratios. To account for these observational results, we propose a scenario based on a toy model of halted pebble drift relevant to the pebble-rich disks. We show that a volatile C/O ratio close to unity and low elemental C and O abundance can be achieved in inner disks only if (1) about 95% of the icy grains are blocked in the outer disk, (2) the outer disk is chemically evolved to reach elevated gas-phase C/O and low (C+O)/H abundance ratio, and (3) the gas in the outer disk had time to be advected to the inner disk. In this scenario, DL Tau and perhaps V1094 Sco would be the rare examples for which all these conditions are met. Therefore, a high C ₂ H ₂ /H ₂ O flux ratio in pebble-rich disks would have a different origin than that proposed for very low-mass stars, for which fast drift of oxygen-rich pebbles would eventually leave a carbon-rich inner disk. Destruction of carbonaceous grains appears to be a less compelling scenario to account for the high C ₂ H ₂ /H ₂ O flux ratio in DL Tau and V1094 Sco because existing models require strong pebble drift and low accretion rates to enrich the inner disk in volatile carbon. Alternatively, the mid-IR molecular features might be related to the low optical depth of the dust, which would reveal a normally hidden reservoir of C ₂ H ₂ deeper down to the midplane. Finally, we show for the first time that the disks with a high C ₂ H ₂ /H ₂ O flux ratio exhibit a prominent silica dust component, a result found in four disks published so far (V1094 Sco, DL Tau, CY Tau, and DoAr 33). We propose that the re-formation of dust at the sublimation front of silicates in a gas with super-solar (but below unity) C/O ratio leads to a silica stoichiometry (SiO ₂ ). In turn, silica constitutes a promising diagnostic of the C/O ratio in the inner disks. Conclusions . The diversity of inner disks as revealed by mid-IR spectroscopy might be driven by the joint evolution of the outer versus inner disk via chemical evolution of the cold outer disk and radial redistribution of gas and ice-coated dust. Studies gathering large samples and forward modeling are required to confirm this proposal and obtain a global picture of disk evolution.

Context . Silicates are key constituents of planet-forming disks and are among the most important building blocks of rocky planets. Mid-infrared spectral features of micron-sized silicate grains are powerful tracers of grain growth, mineralogy, and disk chemistry. Aims . We characterized the dust mineralogy in T Tauri disks using James Webb Space Telescope (JWST)/Mid-Infrared Instrument (MIRI) observations. A further aim of ours was to investigate the connections between the dust and molecular gas compositions. Methods . We analyzed JWST/MIRI spectra of 26 disks as part of the MIRI mid-Infrared Disk Survey (MINDS). We employed spectral decomposition with our new DustComp tool to derive the mass fractions of individual dust species. We included in our fits Mg ₂ SiO ₄ (forsterite), MgSiO ₃ (enstatite), and SiO ₂ (silica) together with amorphous silicates of corresponding stoichiometry. Results . We find that Mg-rich (and Fe-poor) silicates represent our data well. Fit residuals are typically within ±3%. Grain size distributions are skewed toward larger sizes (>2 μm), indicating significant growth. Large (~5 μm-sized) amorphous Mg-silicates were robustly detected, whereas the presence of large crystalline grains could not be firmly established. The average dust composition is dominated by grains of Mg ₂ SiO ₄ stoichiometry (~60%, including amorphous and crystalline state), followed by MgSiO ₃ (~30%) and SiO ₂ (∼10%). The mass fractions of crystalline grains are typically in the 5-24% range, with a mean of 14%. We robustly detected annealed silica in nine objects, with cristobalite as the main polymorph. We found a correlation between dust and molecular gas composition: disks with strong annealed silica features show relatively strong CO ₂ emission, while forsterite-rich disks display stronger H ₂ O emission. Disks with annealed silica features may also have elevated gas-phase C/O ratios, suggesting a process, such as dust sublimation and recondensation, that establishes thermo-chemical equilibrium between solids and gas. Conclusions . The correlation between dust and gas may provide the first indication that the molecular gas composition regulates the availability of dust species in the inner disk.

2MASS J04381486+2611399 (or J0438) is one of the few young brown dwarfs (BDs) with a highly inclined ( i ∼ 70°) disk. Here we report results from JWST Mid-Infrared Instrument (MIRI) Medium Resolution Spectroscopy, Hubble Space Telescope (HST) Advanced Camera for Surveys, and Atacama Large Millimeter/submillimeter Array (ALMA) Band 7 observations. Despite its late spectral type (M7.25), the spectrum of J0438 resembles those of inner disks around earlier-type stars (K1–M5, T Tauri stars), with a volatile reservoir lacking hydrocarbons (except for acetylene, C ₂ H ₂ ) and dominated by water. Other identified species are H ₂ , CO ₂ , HCN, [Ar ⁺ ], and [Ne ⁺ ]. The dominance of water over hydrocarbons is driven by multiple factors such as disk dynamics, young disk age, low accretion rate, and possible inner disk clearing. J0438 appears highly dynamic, showing a seesaw-like variability and extended emission in H ₂ S (1), S (3), S (5), [Ne ⁺ ], and CO ( J = 3–2). Interestingly, the CO emission reaches up to 400 au from the BD, suggesting ongoing infalling/outflowing activity impacting the disk chemistry. These observations underscore the combined power of MIRI, HST, and ALMA in characterizing the chemical diversity and dynamics of BD disks.

Crystalline silicates form at high temperatures (>900 K) (refs. ^1,2). Their presence in comets^3,4,5,6 suggests that high-temperature dust processing occurred in the early Solar System and was subsequently transported outwards to comet-forming regions. However, direct evidence for this crystallization and redistribution in Sun-like protostars has remained unknown. By comparing James Webb Space Telescope mid-infrared spectra of the periodically bursting protostar EC 53 (ref. ⁷), we detect crystalline silicate (forsterite and enstatite) emission features that appear only during the burst. The emergence of these features indicates active crystal formation by thermal annealing in the hot inner disk during the accretion burst. We also detect a nested outflow—a collimated atomic jet enclosed by slower molecular outflows, consistent with magnetohydrodynamic wind models⁸. This configuration provides a mechanism for the outward transport of freshly crystallized silicates⁹. To our knowledge, our results provide the first direct observational evidence of in situ silicate crystallization during episodic accretion bursts in a very young star still embedded in its dense envelope. Although we do not directly detect grains transported to the outer disk, the observed trends are consistent with outward redistribution, indicating that both dust processing and transport occur during the earliest and most dynamic stages of star formation.

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 (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.

Context. The chemistry of disks around brown dwarfs (BDs) remains largely unexplored due to their faintness. Despite the efforts performed with Spitzer, we have far less understanding of planet formation, chemical composition, disk structure, and evolution in disks around BDs compared to their more massive counterparts (T Tauri and Herbig Ae/Be stars), which are more readily studied due to their greater brightness. Recent JWST observations, with up to an order of magnitude improvement in both spectral and spatial resolution, have shown that these systems are chemically rich, offering valuable insights into giant planet formation. Aims. As part of the MIRI mid-INfrared Disk Survey (MINDS) JWST guaranteed time program, we aim to characterize the gas and dust composition of the disk around the brown dwarf [NC98] Cha HA 1, hereafter Cha H α 1, in the mid-infrared. Methods. We obtained data from the MIRI Medium Resolution Spectrometer (MRS) from 4.9 to 28 μm ( R ∼ 1500–3500; FWHM ∼ 0.2″–1.2″). We used the dust fitting tool DuCK to investigate the dust composition and grain sizes, while we identified and fit molecular emission in the spectrum using slab models. Results. Compared with disks around very low mass stars, clear silicate emission features are seen in this BD disk. In addition, JWST reveals a plethora of hydrocarbons, including C ₂ H ₂ , ¹³ CCH ₂ , CH ₃ , CH ₄ , C ₂ H ₄ , C ₄ H ₂ , C ₃ H ₄ , C ₂ H ₆ , and C ₆ H ₆ which suggest a disk with a gas C/O > 1. Additionally, we detected CO ₂ , ¹³ CO ₂ , HCN, H ₂ , and H ₂ O. Notably, CO and OH are absent from the spectrum. The dust is dominated by large ∼4 μm size amorphous silicates (MgSiO ₃ ). We inferred a small dust mass fraction (> 10%) of 5 μm size crystalline forsterite. We did not detect any polycyclic aromatic hydrocarbons. Conclusions. The mid-infrared spectrum of Cha H α 1 shows the most diverse chemistry seen to date in a BD protoplanetary disk, consisting of a strong dust feature, 12 carbon-bearing molecules plus H ₂ , and water. The diverse molecular environment offers a unique opportunity to test our understanding of BD disk chemistry and how it affects the possible planets forming in them.

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 FC₂H₂/FH₂O 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 FC₂H₂/FH₂O, 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.

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.

Mechanisms such as collisions of rocky bodies or cometary activity give rise to dusty debris disks. Debris disks trace the leftover building blocks of planets, and thus also planetary composition. HD 172555, a stellar twin of β Pic, hosts a debris disk thought to have resulted from a giant collision. It is known for its extreme mid-infrared silica dust feature, indicating a warm population of silica-rich grains in the asteroid belt (∼5 au), cold CO observed by Atacama Large Millimeter/submillimeter Array (ALMA), and small bodies evaporating as they approach close to the star. Our JWST Mid-InfraRed Instrument/Medium Resolution Spectroscopy (MIRI MRS) observations now reveal emission from an inner gaseous disk (β Pic, is also detected. All lines are broadened by Keplerian rotation, pinpointing the gas location. The HD 172555 system serves as a unique laboratory to study the composition of planetesimals, asteroids, and comets beyond the solar system. The comparison to β Pic reveals that the gas in HD 172555 is hotter, closer to the star, and poor in argon—suggesting it originates from evaporating rocky bodies near the star, while β Pic’s gas may trace volatile-rich bodies from larger separations.