28 research outputs found
Warm chemistry in planet-forming disks
No full textTerrestrial planets like Earth form in the inner regions (>10 au) of planet-forming disks around young stars. Understanding how the composition of these planets links to the disk’s material is key to evaluating Earth’s uniqueness. This thesis investigates the warm chemistry in these regions critical for terrestrial planet formation using mid-infrared observations, particularly with the James Webb Space Telescope (JWST), and advanced modelling.The thermo-chemical radiation code ProDiMo was used to simulate disks and develop an extended hydrocarbon network capable of forming species up to C8H5+. While this network subtly affected hydrocarbon spectra, such as C2H2, it improved understanding of inner disk chemistry. JWST observations revealed hydrocarbons like C6H6, C2H2, C4H2, CH3, CH4, and C2H6 in the disk around Sz28, indicating carbon-dominated chemistry in inner regions with a C/O>1. 2D disk modelling utilising the extended hydrocarbon chemical network concluded that these hydrocarbons can form by the gas-phase chemistry in the observable surface layers of the disk when the elemental C/O>1. 2D modelling of another low-mass star disk that has huge column densities of hydrocarbons showed an inner dust-depleted region with C/O>80, transitioning to an outer disk resembling a TTauri star (C/O∼0.45) can explain its observations.To assess thermodynamic equilibrium in dense, warm regions, a new chemical network, ChaiTea combines exoplanet and disk chemistry with updated thermochemical data. Models showed equilibrium was disrupted by UV radiation and cosmic rays, though deviations were minimal in a small, warm, dense zone near the inner rim.This thesis forms a cornerstone in interpreting the JWST observations and transitioning from 0D to full 2D thermo-chemical disk models
Can thermodynamic equilibrium be established in planet-forming disks?
No full textContext. The inner regions of planet-forming disks are warm and dense. The chemical networks used for disk modelling so far were developed for a cold and dilute medium and do not include a complete set of pressure-dependent reactions. The chemical networks developed for planetary atmospheres include such reactions along with the inverse reactions related to the Gibb’s free energies of the molecules. The chemical networks used for disk modelling are thus incomplete in this respect.
Aims. We want to study whether thermodynamic equilibrium can be established in a planet-forming disk. We identify the regions in the disk most likely to reach thermodynamic equilibrium and determine the timescale over which this occurs.
Methods. We employ the theoretical concepts used in exoplanet atmosphere chemistry for the disk modelling with PROtoplanetary DIsk MOdel (PR
Characterizing the Discovery of a New Trans-Neptunian Object Binary in a Trailed Point-spread Function Search
No full textThe Latitude Density Search utilized the Hyper Suprime-Cam on the Subaru Telescope to discover 60 moving objects in the outer solar system, 54 of which have semimajor axes beyond 30 au. The images were acquired in exceptional seeing (0.″4) and reached a detection limit of m r ≃ 25.2. The two night arcs were used to calculate orbits that are poorly constrained; however, the distance and inclination are the parameters best constrained by short arcs, and a reasonable determination can be made of which objects are cold classical trans-Neptunian objects (TNOs) and which are dynamically excited. We identify 10 objects as likely cold classical objects. We searched all of the detections for binary sources using a trailed point-spread function (PSF) subtraction method and identified one binary object with a separation of 0.″34 and a secondary with 17% the brightness of the primary (2.0 magnitudes fainter). This is the brightest TNO in the sample, the previously known object (471165) 2010 HE79, which has a dynamically excited orbit. Because of the excellent seeing, this search was sensitive to secondaries with 0.″34 separation and a brightness of ≥50% the primary brightness for seven objects, including one cold classical. This gives an intrinsic binary fraction of 17−10+19 % (one of six) for the dynamically excited objects within our detection limits. The trailed PSF subtraction method used in the Latitude Density Search to identify binaries, fit the two components, and determine the sensitivity limits is a useful tool that could be more broadly applied to identify binary TNOs and track known binary TNO orbits
MINDS:The very low-mass star and brown dwarf sample: II. Probing disk settling, dust properties, and dust-gas interplay with JWST/MIRI
No full textContext. 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.</p
Hydrocarbon chemistry in the inner regions of planet-forming disks
No full textThe analysis of the mid-infrared spectra helps understanding the composition
of the gas in the inner, dense and warm terrestrial planet forming region of
disks around young stars. ALMA has detected hydrocarbons in the outer regions
of the planet forming disk and Spitzer detected \ce{C2H2} in the inner regions.
JWST- MIRI provides high spectral resolution observations of \ce{C2H2} and a
suite of more complex hydrocarbons are now reported. Interpreting the fluxes
observed in the spectra is challenging and radiation thermo-chemical codes are
needed to properly take into account the disk structure, radiative transfer,
chemistry and thermal balance. Various disk physical parameters like the
gas-to-dust ratio, dust evolution including radial drift, dust growth and
settling can affect the fluxes observed in the mid-IR. Still, thermo-chemical
disk models were not always successful in matching all observed molecular
emission bands simultaneously. The goal of this project is two-fold. We analyse
the warm carbon chemistry in the inner regions of the disk, i.e. within 10 au
to find pathways forming \ce{C2H2} potentially missing from the existing
chemical networks. Second, we analyse the effect of the new chemistry on the
line fluxes of acetylene. We use radiative thermo-chemical disk code {P{\small
RO}D{\small I}M{\small O}} to expand the hydrocarbon chemistry that occurs in a
typical standard T Tauri disks. We used the UMIST and the KIDA rate databases
for collecting reactions for the species. We include a number of three-body and
thermal decomposition reactions from STAND2020 network. We included isotopomers
for the species that were present in the databases. The chemistry is then
analysed in the regions that produce observable features in the mid-infrared
spectra. The effect of expanding the hydrocarbon chemistry on the mid-infrared
spectra is studied. Acetylene is formed via two ....Comment: accepted for publication in A&
MINDS: Strong oxygen depletion in the inner regions of a very low-mass star disk?
No full textContext. Thanks to JWST, a plethora of species in planet-forming disks around very low mass stars such as C2H2, C6H6, C4H2, CH3 etc. are being discovered. The column densities of these species retrieved from 0D slab models are very large (e.g. of the order of 1020 cm−2). This indicates a carbon-dominated chemistry in a gas with a high C/O ratio. The disk around 2MASS-J1605321-1993159 (M4.5) is one such source showing a molecular pseudo-continuum of C2H2. Notably, two oxygen-bearing molecules, CO and CO2, are also detected in this source.
Aims. We aim to take the next step beyond 0D slab models to interpret the spectrum. We examine whether 2D thermo-chemical disk models can produce the large inferred column densities of C2H2 in the inner regions of the disk and produce a pseudo-continuum in the mid-IR spectrum. We also seek to constrain whether the depletion of oxygen or the enrichment of carbon causes the high C/O ratio triggering a carbon-dominated chemistry.
Methods. We utilised the radiative thermo-chemical disk model PRODIMO to identify a disk structure that is capable of producing the observed molecular emission of species such as CO, CO2, C2H2, and H2O simultaneously. The spectrum was generated using the fast line tracer FLiTs. We derived the gas temperature ⟨T⟩, column density ⟨log10N⟩, and the emitting area ⟨r1 − r2⟩ for these molecules from the 2D disk model and compared them to the parameters retrieved originally from 0D slab models. We used the different effect that changing the O or C abundance has on CO and C2H2, respectively to discriminate between O depletion and C enhancement.
Results. We find that a disk structure characterised by the presence of a gap can best explain the observations. The inner disk is strongly depleted in dust, especially small grains (<5 µm), and elemental oxygen, leading to a large C/O ratio. This is required to produce a molecular pseudo-continuum of C2H2 and at the same time a relatively weak CO emission. The P- and R-branch of C2H2 probe deeper layers of the disk whereas the Q-branch probes mostly the surface layers. The combined emission of CO and CO2 puts strong constraints on the gap’s location (0.1–0.5 au) given a disk gas mass. We also report a new detection of the CO ν= 2→1 transition in the JWST spectrum.
Conclusions. Two-dimensional thermo-chemical disk models are able to produce the observed molecular pseudo-continuum of C2H2. We find that the combination of different species emission in the JWST spectra can be used to discriminate between different scenarios such as O-depletion, C-enhancement or both, and offers the potential to extract spatial substructure at scales smaller than ∼1 au
MINDS
No full textContext. The majority of young stars form in multiple systems, the properties of which can significantly impact the evolution of any circumstellar disks.
Aims. We investigate the physical and chemical properties of the equal-mass, small-separation (~66 milliarcsecond, ~9 au) binary system DF Tau. Previous spatially resolved observations indicate that only DF Tau A has a circumstellar disk, while DF Tau B does not, as concluded by a lack of accretion signatures and a near-infrared excess.
Methods. We present JWST-MIRI MRS observations of DF Tau. The MIRI spectrum shows emission from a forest of H2O lines and emission from CO, C2H2, HCN, CO2, and OH. Local thermodynamic equilibrium slab models were used to determine the properties of the gas. The binary system is not spatially or spectrally resolved in the MIRI observations; therefore, we analyzed high spatial and spectral resolution observations from ALMA, VLTI-GRAVITY, and IRTF-iSHELL to aid in the interpretation of the molecular emission observed with JWST.
Results. The 1.3 mm ALMA observations show two equal-brightness sources of compact (R ≲ 3 au) continuum emission that are detected at high significance, with separations consistent with astrometry from VLTI-GRAVITY and movement consistent with the known orbital parameters of the system. We interpret this as a robust detection of the disk around DF Tau B, which we suggest may host a small (~1 au) cavity; such a cavity would reconcile all of the observations of this source. In contrast, the disk around DF Tau A is expected to be a full disk, and spatially and spectrally resolved dust and gas emission traced by ground-based infrared observations point to hot, close-in (≲0.2 au) material around this star. High-temperature emission (~500–1000 K) from H2O, HCN, and potentially C2H2 in the MIRI data likely originates in the disk around DF Tau A, while a cold H2O component (≲200 K) with an extended emitting area is consistent with an origin from both disks.
Conclusions. Given the unique characteristics of this binary pair, complementary observations are critical for constraining the properties of these disks. Despite the very compact outer disk properties, the inner disk composition and the conditions of the DF Tau disks are remarkably similar to those of isolated systems, suggesting that neither the outer disk evolution nor the close binary nature are driving factors in setting the inner disk chemistry in this system. However, constraining the geometry of the disk around DF Tau B, via higher angular resolution ALMA observations for instance, would provide additional insight into the properties of the mid-infrared gas emission observed with MIRI. JWST observations of spatially resolved binaries, at a range of separations, will be important for understanding the impact of binarity on inner disk chemistry more generally
MINDS Cha Hα 1, a brown dwarf with a hydrocarbon-rich disk
No full textContext. 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 Hα 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 C2H2 13CCH2, CH3, CH4, C2H4, C4H2, C3H4, C2H6, and C6H6 which suggest a disk with a gas C/O > 1. Additionally, we detected CO2 13CO2, HCN, H2, and H2O. Notably, CO and OH are absent from the spectrum. The dust is dominated by large ∼4 μm size amorphous silicates (MgSiO3). 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 Hα 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 H2, 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.</p
Erratum:“MINDS. JWST-MIRI Observations of a Spatially Resolved Atomic Jet and Polychromatic Molecular Wind toward SY Cha” (2025, ApJ, 980, 148)
No full textThe published article contained several errors in the calculation of the annular flux and the total H2 mass. Table 2 gives the updated annular flux values. Additionally, the rotation diagram analysis does not account for the normalization used in the partition function of A. Popovas & U. G. Jørgensen (2016), which requires also normalizing the degeneracies by a factor of 0.25. The S(8) line is excluded from the rotation diagram fit due to contamination from the overlapping CO emission. Correcting for these errors results in a lower temperature for the hot H2 component, a lower column density for both the hot and the warm component, and a lower H2 mass. Table 3 gives the updated properties derived from the rotation diagram fits shown in Figure 4. This results in an order of magnitude decrease in the derived H2 wind mass loss rate, which has been updated to 3.77 ± 0.63 × 10−10M⊙ yr−1 from the previous derived rate of 3 ± 2 × 10−9 M⊙ yr−1 . This lower rate is within the expected range (relative to the stellar accretion rate) for an MHD wind, as well as a photoevaporative wind. (Table presented) (Figure presented) (Table presented).</p
MINDS: The DR Tau disk: I. Combining JWST-MIRI data with high-resolution CO spectra to characterise the hot gas
No full textISSN:0004-6361ISSN:1432-0746ISSN:1432-074
