Yvonne J. Pendleton

Trans-Neptunian objects (TNOs) harbor pristine materials from the early solar system. The James Webb Space Telescope offers major insights into the spectral classification of these small bodies. Among the three major spectral classes of TNOs, “cliff” types exhibit steep, red slopes from 0.7 to 1.2 μ m, sheer reflectance drops from 2.65 to 2.85 μ m, and deep 3 μ m absorptions with low reflectance longward of the band. These types show more features associated with simple hydrocarbons and methanol than other TNO types. Cliff-type spectra show two further spectral subclasses (hereafter Cliff1 and Cliff2 types), with all cold-classical TNOs exhibiting Cliff2-type spectra. We apply radiative-transfer-based spectral modeling from 2.0 to 3.6 μ m for 16 cliff-type TNOs to quantify compositions and investigate depth dependencies, which is crucial for understanding the cliff type’s role in the outer solar system’s development. We find mixtures of methanol, water, and tholins/organics are primarily responsible for the prototypical 3 μ m region of cliff-type spectra, with methanol and tholins/organics being particularly suggestive of distant formation. Our models also demonstrate Cliff1 and Cliff2 types have distinct compositions of higher methanol and tholin/organic abundances, respectively. We find the Mors-Somnus binary system to be compositionally homogenous, consistent with a formation scenario that sampled a similar region in the protoplanetary disk (i.e., streaming instability). Object 2004 PG ₁₁₅ is compositionally distinct from the rest of the cliff types, suggesting some objects have compositions intermediate between cliff subclasses and major TNO classes. Observations with higher spectral resolution and signal-to-noise ratio are crucial for better understanding minor, often aliphatic components of cliff-type spectra.

C≡N-bearing species are valuable tracers of nitrogen chemistry in the solar system. Ranging from volatile compounds like HCN and CH ₃ CN in cometary comae to refractory CN-bearing organics in some micrometeorites and interplanetary dust particles, these species originate in, or are derived from, molecular precursors in the interstellar medium (ISM). The icy surfaces of medium-sized trans-Neptunian objects (TNOs) represent an intermediate stage in the chemical evolution and incorporation of N-bearing molecules from the ISM into the solar system. Herein, we investigate the nature, distribution, and origin of CN-bearing species on several TNOs and one centaur observed within the James Webb Space Telescope DiSCo-TNOs program. We find a spectral feature at 4.62 μm consistent in band position and width as OCN ⁻ observed in the ISM and the laboratory. This feature is most pronounced on TNOs in the Cliff2 group (Cliff2-TNOs, a compositional subgroup characterized by low ice content that include the cold classical TNOs, suggesting they formed farthest from the Sun) and might be tracing an original chemical inventory in the protoplanetary disk. Cliff2-TNOs are also the best candidates for studying the contribution of refractory C≡N-containing organics to a broad 4.5 µm spectral feature. Finally, a close spectral resemblance between Cliff2-TNOs and the centaur in our analysis, which exhibits both the 4.62 and 4.5 μm bands, provides a plausible dynamical channel that can enrich the inner solar system with N-bearing species. Our work advances the understanding of N-bearing species in the solar system and their evolutionary history beginning in the ISM.

Infrared spectra of hydrocarbon dust absorption bands toward the bright hypergiant Cygnus OB2-12 are compared to published spectra of the Quintuplet Cluster, a sightline to the Galactic center. The Cyg OB2-12 data include a new ground-based 2.86−3.70 μm spectrum and a previously published, but here further analyzed, spectrum of the 5.50–7.34 μm region. Higher-spectral-resolution data for the Cyg OB2-12 sightline in the 3 μm region allows a detailed comparison of the 3.4 μm aliphatic bands to those observed toward the Quintuplet. Despite differences in interstellar environments along each sightline, strong similarities are observed in the central wavelengths and relative strengths for bands at ∼3.3, 3.4, 5.85, 6.2, and 6.85 μm. Analysis of these bands, produced by aromatic, aliphatic, olefinic, hydrogenated, and oxygenated components, shows that carbonaceous dust is a significant component of the diffuse interstellar medium (ISM), second in abundance only to silicates, and is primarily aromatic in nature. The grains producing these bands likely consist of large aromatic carbon cores with thin aliphatic mantles composed of hydrogenated amorphous carbon. Laboratory analog spectra reproduce the observed aliphatic absorption bands well, supporting the presence of such mantles. We present evidence that the carriers of both the 3.4 μm aliphatic and the 3.3 μm aromatic bands reside exclusively in the diffuse ISM, and that the 3.3 μm bands observed in the diffuse ISM differ from the 3.25 μm band seen in dense clouds, implying chemically distinct carriers.

The dusty interstellar medium (ISM) of the Milky Way is distributed in a complex, cloudy structure. It is fundamental to the radiation balance within the Milky Way, provides a reaction surface to form complex molecules, and is the feedstock for future generations of stars and planets. The life cycle of interstellar dust is not completely understood, and neither are its structure nor composition. The abundance, composition, and structure of dust in the diffuse ISM can be determined by combining infrared, optical, and ultraviolet spectroscopy. JWST enables measurement of the faint absorption of ISM dust grains against bright stars at kiloparsec distances across the infrared spectrum. Here we present an overview of the project “Webb Investigation of Silicates, Carbons, and Ices” (WISCI) along with interpretation of two targets, GSC 08152-02121 and CPD-59 5831. Observations of 12 WISCI target stars were taken by JWST, the Hubble Space Telescope, Himalayan Chandra Telescope, and the Very Large Telescope. We use these to characterize the targets’ spectral types and calculate their line-of-sight extinction parameters, A _V and R _V. We find absorption in the JWST spectra of GSC 08152-02121 and CPD-59 5831 associated with carbonaceous dust around 3.4 and 6.2 μm and amorphous silicates at 9.7 μm. In GSC 08152-02121, we also find indications of absorption by trapped water around 3 μm. This first look from WISCI demonstrates the line-of-sight variability within the sample, and the program’s potential to identify and correlate features across ultraviolet to mid-infrared wavelengths.

Context. Spectra of the nearly edge-on protoplanetary discs observed with the JWST have shown ice absorption bands of varying optical depths and peculiar profiles, challenging radiative transfer modelling and our understanding of dust and ice in discs. Aims. With the aim of constraining the underlying disc’s structure and evolutionary state, we build models including dust grain size, shape, and composition to reproduce JWST IFU spectroscopy of a well-characterised, massive, and large edge-on disc, Tau 042021. Specifically, we aim to match its spectral energy distribution, the spatial distribution of the dust and ice, and the spectral characteristics of the dust continuum and ice bands profiles, as well as test for the presence of astronomical polycyclic aromatic hydrocarbons (PAH) band carriers. Methods. We explored radiative transfer models using different dust grain size distributions, including grains with effective radii of a_eff = 0.005–3000 μm. Mass absorption and scattering coefficients for distributions of triaxial ellipsoidal grains were calculated using the discrete dipole approximation (DDA) for small size parameters (2πa_eff/λ < 10), whereas the hollow sphere approximation was used for larger size parameters. We considered compositions including silicates, amorphous carbon, and mixtures of water, carbon dioxide, and carbon monoxide. The resulting orientation-averaged scattering matrices were input into RADMC-3D Monte Carlo radiative transfer models of Tau 042021 to simulate the spectral cubes observed with JWST-NIRSpec and MIRI. We compared the calculated optical depth distributions and profiles of the main ice bands to observations, including water at 3.05 μm, carbon monoxide at 4.67 μm, and carbon dioxide at 4.26 μm. We also compared these results to archival JWST-NIRCam and ALMA continuum images. We tested three increasingly complex disc structures, starting from a standard model and adding first an extended atmosphere, then a disc wind containing astro-PAHs. Results. The observed near- to mid-infrared spectral energy distribution requires efficient scatterers, implying dust distributions that include grains of up to several tens of microns in size. The intensity distribution perpendicular to the disc exhibits emission profile wings extending into the upper disc atmosphere at altitudes exceeding the classical scale height expected in the isothermal hydrostatic limit. We produce ice absorption images that demonstrate the presence of icy dust grains up to altitudes high above the disc midplane, more than three hydrostatic equilibrium scale heights. We demonstrate the presence of a wind containing the carriers of astronomical PAH bands. The wind appears as an X-shaped emission at 3.3, 6.2, 7.7, and 11.3 μm, characteristic wavelengths associated with the infrared astronomical PAH bands. We associate the spatial distribution of this component with carriers of astronomical PAH bands that form a layer of emission at the interface with the H₂ wind.

Context. Recent JWST observations have measured the ice chemical composition towards two highly extinguished background stars, NIR38 and J110621, in the Chamaeleon I molecular cloud. The observed excess of extinction on the long-wavelength side of the H₂O ice band at 3 μm has been attributed to a mixture of CH₃OH with ammonia hydrates NH₃·H₂O), which suggests that CH₃OH ice in this cloud could have formed in a water-rich environment with little CO depletion. Laboratory experiments and quantum chemical calculations suggest that CH₃OH could form via the grain surface reactions CH₃ + OH and/or C + H₂O in water-rich ices. However, no dedicated chemical modelling has been carried out thus far to test their efficiency. In addition, it remains unexplored how the efficiencies of the proposed mechanisms depend on the astrochemical code employed. Aims. We modelled the ice chemistry in the Chamaeleon I cloud to establish the dominant formation processes of CH₃OH, CO, CO₂, and of the hydrides CH₄ and NH₃ (in addition to H₂O). By using a set of state-of-the-art astrochemical codes (MAGICKAL, MONACO, Nautilus, UCLCHEM, and KMC simulations), we can test the effects of the different code architectures (rate equation vs. stochastic codes) and of the assumed ice chemistry (diffusive vs. non-diffusive). Methods. We consider a grid of models with different gas densities, dust temperatures, visual extinctions, and cloud-collapse length scales. In addition to the successive hydrogenation of CO, the codes’ chemical networks have been augmented to include the alternative processes for CH₃OH ice formation in water-rich environments (i.e. the reactions CH₃ + OH → CH₃OH and C + H₂O → H₂CO). Results. Our models show that the JWST ice observations are better reproduced for gas densities ≥10⁵ cm⁻³ and collapse timescales ≥10⁵ yr. CH₃OH ice formation occurs predominantly (>99%) via CO hydrogenation. The contribution of reactions CH₃ + OH and C + H₂O is negligible. The CO₂ ice may form either via CO + OH or CO + O depending on the code. However, KMC simulations reveal that both mechanisms are efficient despite the low rate of the CO + O surface reaction. CH₄ is largely underproduced for all codes except for UCLCHEM, for which a higher amount of atomic C is available during the translucent cloud phase of the models. Large differences in the predicted abundances are found at very low dust temperatures (T_dustdust. This could explain the rather constant ice chemical composition found in Chamaeleon I and other dense cores despite the different visual extinctions probed.

In the coldest regions of molecular clouds, carbon and oxygen are incorporated into icy dust grains. Despite its outsized role in star and planet formation, sequential formation of ice is poorly constrained. Infrared spectroscopy probes ice chemistry, but previous telescopes observed insufficient lines of sight to map a single cloud. Here we present cospatial maps of H₂O, CO₂ and CO ice over the central region of the Chamaeleon I molecular cloud, using 44 lines of sight observed with the James Webb Space Telescope. Correlations at column densities ten times larger than previous work suggest additional CO₂ ice formation in CO ice for the densest lines of sight. This large statistical sampling within a single cloud represents a step change in ice mapping, eliminating averaging over clouds with different intrinsic chemical environments. Mapping opens the door to probing gas–grain exchanges, snow lines and chemical evolution in the densest regions and drawing conclusions on the impact of ice chemistry on wider astrophysics.

The spectral diversity of trans-Neptunian objects (TNOs) is crucial for understanding the processes that led to the formation and evolution of planetesimals in the outer solar system. Using near-IR (NIR) spectra obtained by the James Webb Space Telescope (JWST) as part of the DiSCo-TNOs large program, we report the detection of well-clustered subgroups of TNOs. A first subgroup has strong NIR features with contributions from H₂O, CO₂, CO, CH₃OH, and other organic molecules. The 2.27 μm band area, commonly attributed to methanol, decreases with increasing eccentricity, which is compatible with a late destruction of CH₃OH by cosmic ion irradiation at the edge of the heliosphere. The absence of the strongest CH₃OH bands in the JWST spectra is compatible with an irradiation-induced surface stratification, with CH₃OH abundance increasing with increasing depth. A second subgroup has much weaker NIR bands, and these cannot be explained by a late irradiation scenario. This group is further divided into two subgroups (cold classical TNOs and objects with low perihelion) that are spectrally very similar except for their CO₂ band area. We propose two possible interpretations. In one scenario, the TNO subgroups sampled a similar molecular inventory in the protoplanetary disk, after which early surface processes, such as primordial sublimation or irradiation from the young Sun, sculpted the two groups before planetary migration occurred. In a second scenario, the subgroups formed in different locations of the disk where molecules were available in different abundances. A combination of both scenarios is also possible.

Context. The James Webb Space Telescope large program DiSCo-TNOs has recently shown that CO₂ ice is ubiquitous on 54 mediumsize trans-Neptunian objects (TNOs). TNO surfaces are found to define three main spectral and thus compositional groups that are likely linked to their position before planetary migration. CO ice is observed on the spectral type that is richest in CO₂ and on the type that is richer in CH₃OH and organics. Considerations on the thermal evolution of TNOs predicted the depletion of hypervolatiles such as CO from their surface layers, however. Aims. We investigate a potential irradiation origin of CO as well as its stability by studying the distribution of CO in two TNO compositional types and compared it with irradiation experiments. Methods. We studied the 4.68 µm band of CO and the 2.70 µm band of CO₂ to probe the relation between the two molecules in 33 TNOs. We performed ion irradiation experiments on CO₂ and CH₃OH ices at 45 and 60 K with 30 keV H⁺ . We compared the laboratory spectra to TNO observations by focusing on the band areas and positions. Results. We find that the two types of surfaces in which CO is detected are very distinct in terms of their relative abundances and chemical environment. CO that is observed on surfaces that are rich in CO₂ are consistent with being produced by CO₂ irradiation, specifically, at 45 K. On objects that are rich in CH₃OH and complex organics, CO is more likely formed by irradiation of CH₃OH. As the CO band areas are only partly related with temperature, the chemical environment plays a major role in the CO retention. Conclusions. We find that the CO that is observed on TNO surfaces is compatible with being a secondary molecule that is entirely formed by late irradiation processes. Its abundance and stability is mostly controlled by the matrix from which it formed.