2D disc modelling of the JWST line spectrum of EX Lupi

We introduce a number of new theoretical approaches and improvements to the thermo-chemical disc modelling code ProDiMo to better predict and analyse the JWST line spectra of protoplanetary discs. We develop a new line escape probability method for disc geometries, a new scheme for dust settling, and discuss how to apply UV molecular shielding factors to photorates in 2D disc geometry. We show that these assumptions are crucial for the determination of the gas heating/cooling rates and discuss how they affect the predicted molecular concentrations and line emissions. We apply our revised 2D models to the protoplanetary disc around the T Tauri star EX Lupi in quiescent state. We calculate infrared line emission spectra between 5 and 20 mic by CO, H2O, OH, CO2, HCN, C2H2 and H2, including lines of atoms and ions, using our full 2D predictions of molecular abundances, dust opacities, gas and dust temperatures. We develop a disc model with a slowly increasing surface density structure around the inner rim that can simultaneously fit the spectral energy distribution, the overall shape of the JWST spectrum of EX Lupi, and the main observed molecular characteristics in terms of column densities, emitting areas and molecular emission temperatures, which all result from one consistent disc model. The spatial structure of the line emitting regions of the different molecules is discussed. High abundances of HCN and C2H2 are caused in the model by stellar X-ray irradiation of the gas around the inner rim.Comment: accepted by A&A, 25 pages, 15 figures, 7 table

The diverse chemistry of protoplanetary disks as revealed by JWST

Early results from the JWST-MIRI guaranteed time programs on protostars (JOYS) and disks (MINDS) are presented. Thanks to the increased sensitivity, spectral and spatial resolution of the MIRI spectrometer, the chemical inventory of the planet-forming zones in disks can be investigated with unprecedented detail across stellar mass range and age. Here data are presented for five disks, four around low-mass stars and one around a very young high-mass star. The mid-infrared spectra show some similarities but also significant diversity: some sources are rich in CO2, others in H2O or C2H2. In one disk around a very low-mass star, booming C2H2 emission provides evidence for a ``soot'' line at which carbon grains are eroded and sublimated, leading to a rich hydrocarbon chemistry in which even di-acetylene (C4H2) and benzene (C6H6) are detected (Tabone et al. 2023). Together, the data point to an active inner disk gas-phase chemistry that is closely linked to the physical structure (temperature, snowlines, presence of cavities and dust traps) of the entire disk and which may result in varying CO2/H2O abundances and high C/O ratios >1 in some cases. Ultimately, this diversity in disk chemistry will also be reflected in the diversity of the chemical composition of exoplanets.Comment: 17 pages, 8 figures. Author's version of paper submitted to Faraday Discussions January 18 2023, Accepted March 16 202

Water in the terrestrial planet-forming zone of the PDS 70 disk

Terrestrial and sub-Neptune planets are expected to form in the inner ($<10~$AU) regions of protoplanetary disks. Water plays a key role in their formation, although it is yet unclear whether water molecules are formed in-situ or transported from the outer disk. So far Spitzer Space Telescope observations have only provided water luminosity upper limits for dust-depleted inner disks, similar to PDS 70, the first system with direct confirmation of protoplanet presence. Here we report JWST observations of PDS 70, a benchmark target to search for water in a disk hosting a large ($\sim54~$AU) planet-carved gap separating an inner and outer disk. Our findings show water in the inner disk of PDS 70. This implies that potential terrestrial planets forming therein have access to a water reservoir. The column densities of water vapour suggest in-situ formation via a reaction sequence involving O, H$_2$, and/or OH, and survival through water self-shielding. This is also supported by the presence of CO$_2$ emission, another molecule sensitive to UV photodissociation. Dust shielding, and replenishment of both gas and small dust from the outer disk, may also play a role in sustaining the water reservoir. Our observations also reveal a strong variability of the mid-infrared spectral energy distribution, pointing to a change of inner disk geometry.Comment: To appear in Nature on 24 July 2023. 21 pages, 10 figures; includes extended data. Part of the JWST MINDS Guaranteed Time Observations program's science enabling products. Spectra downloadable on Zenodo at https://zenodo.org/record/799102

2D disc modelling of the JWST line spectrum of EXLupi

We introduce a number of improvements to the thermo-chemical disc modelling code ProDiMo and new theoretical approaches that can be used to better predict and analyse the JWST line spectra of protoplanetary discs. We developed a new line escape probability method for disc geometries, and a new scheme for dust settling, and discuss how to apply UV molecular shielding factors to photo rates in 2D disc geometry. We show that these assumptions are crucial for the determination of gas heating and cooling rates and discuss how they affect the predicted molecular concentrations and line emissions. We apply our revised 2D models to the protoplanetary disc around the T Tauri star EXLupi in quiescent state. We calculate infrared line emission spectra between 5 and 20 μm from CO, H2O, OH, CO2, HCN, C2H2, and H2, including lines of atoms and ions, using our full 2D predictions of molecular abundances, dust opacities, and gas and dust temperatures. We developed a disc model with a slowly increasing surface density structure around the inner rim that can simultaneously fit the spectral energy distribution, the overall shape of the JWST spectrum of EXLupi, and the main observed molecular characteristics in terms of column densities, emitting areas, and molecular emission temperatures, which all result from one consistent disc model. The spatial structure of the line-emitting regions of the different molecules is discussed. High abundances of HCN and C2H2 are caused in the model by stellar X-ray irradiation of the gas around the inner rim.</p

2D disc modelling of the JWST line spectrum of EXLupi

We introduce a number of improvements to the thermo-chemical disc modelling code ProDiMo and new theoretical approaches that can be used to better predict and analyse the JWST line spectra of protoplanetary discs. We developed a new line escape probability method for disc geometries, and a new scheme for dust settling, and discuss how to apply UV molecular shielding factors to photo rates in 2D disc geometry. We show that these assumptions are crucial for the determination of gas heating and cooling rates and discuss how they affect the predicted molecular concentrations and line emissions. We apply our revised 2D models to the protoplanetary disc around the T Tauri star EXLupi in quiescent state. We calculate infrared line emission spectra between 5 and 20 μm from CO, H2O, OH, CO2, HCN, C2H2, and H2, including lines of atoms and ions, using our full 2D predictions of molecular abundances, dust opacities, and gas and dust temperatures. We developed a disc model with a slowly increasing surface density structure around the inner rim that can simultaneously fit the spectral energy distribution, the overall shape of the JWST spectrum of EXLupi, and the main observed molecular characteristics in terms of column densities, emitting areas, and molecular emission temperatures, which all result from one consistent disc model. The spatial structure of the line-emitting regions of the different molecules is discussed. High abundances of HCN and C2H2 are caused in the model by stellar X-ray irradiation of the gas around the inner rim.</p

Mixing and diffusion in protoplanetary disc chemistry

We develop a simple iterative scheme to include vertical turbulent mixing and diffusion in PRODIMO thermo-chemical models for protoplanetary discs. The models are carefully checked for convergence towards the time-independent solution of the reaction-diffusion equations, as, for example, used in exoplanet atmosphere models. A series of five TTauri disc models is presented where we vary the mixing parameter αmix from zero to 10−2 and take into account: (a) the radiative transfer feedback of the opacities of icy grains that are mixed upwards; and (b) the feedback of the changing molecular abundances on the gas temperature structure caused by exothermic reactions, and increased line heating and cooling. We see considerable changes in the molecular and ice concentrations in the disc. The most abundant species (H2, CH4, CO, the neutral atoms in higher layers, and the ices in the midplane) are transported both up and down, and at the locations where these abundant chemicals finally decompose, for example by photo processes, the release of reaction products has important consequences for all the other molecules. This generally creates a more active chemistry, with a richer mixture of ionised, atomic, molecular, and ice species, and new chemical pathways that are not relevant in the unmixed case. We discuss the impact on three spectral observations caused by mixing and find that: (i) icy grains can reach the observable disc surface where they cause ice absorption and emission features at IR to far-IR wavelengths; (ii) mixing increases the concentrations of certain neutral molecules observable by mid-IR spectroscopy, in particular OH, HCN, and C2H2; and (iii) mixing can change the optical appearance of CO in ALMA line images and channel maps, where strong mixing would cause the CO molecules to populate the distant midplane

The diverse chemistry of protoplanetary disks as revealed by JWST

Early results from the James Webb Space Telescope-Mid-InfraRed Instrument (JWST-MIRI) guaranteed time programs on protostars (JOYS) and disks (MINDS) are presented. Thanks to the increased sensitivity, spectral and spatial resolution of the MIRI spectrometer, the chemical inventory of the planet-forming zones in disks can be investigated with unprecedented detail across stellar mass range and age. Here, data are presented for five disks, four around low-mass stars and one around a very young high-mass star. The mid-infrared spectra show some similarities but also significant diversity: some sources are rich in CO2, others in H2O or C2H2. In one disk around a very low-mass star, booming C2H2 emission provides evidence for a “soot” line at which carbon grains are eroded and sublimated, leading to a rich hydrocarbon chemistry in which even di-acetylene (C4H2) and benzene (C6H6) are detected. Together the data point to an active inner disk gas-phase chemistry that is closely linked to the physical structure (temperature, snowlines, presence of cavities and dust traps) of the entire disk and which may result in varying CO2/H2O abundances and high C/O ratios &gt;1 in some cases. Ultimately, this diversity in disk chemistry will also be reflected in the diversity of the chemical composition of exoplanets.</p

The diverse chemistry of protoplanetary disks as revealed by JWST

Early results from the James Webb Space Telescope-Mid-InfraRed Instrument (JWST-MIRI) guaranteed time programs on protostars (JOYS) and disks (MINDS) are presented. Thanks to the increased sensitivity, spectral and spatial resolution of the MIRI spectrometer, the chemical inventory of the planet-forming zones in disks can be investigated with unprecedented detail across stellar mass range and age. Here, data are presented for five disks, four around low-mass stars and one around a very young high-mass star. The mid-infrared spectra show some similarities but also significant diversity: some sources are rich in CO2, others in H2O or C2H2. In one disk around a very low-mass star, booming C2H2 emission provides evidence for a “soot” line at which carbon grains are eroded and sublimated, leading to a rich hydrocarbon chemistry in which even di-acetylene (C4H2) and benzene (C6H6) are detected. Together the data point to an active inner disk gas-phase chemistry that is closely linked to the physical structure (temperature, snowlines, presence of cavities and dust traps) of the entire disk and which may result in varying CO2/H2O abundances and high C/O ratios &gt;1 in some cases. Ultimately, this diversity in disk chemistry will also be reflected in the diversity of the chemical composition of exoplanets.</p

Ices in planet-forming disks: Self-consistent ice opacities in disk models

Context. In cold and shielded environments, molecules freeze out on dust grain surfaces to form ices such as H2O, CO, CO2, CH4, CH3OH, and NH3. In protoplanetary disks, such conditions are present in the midplane regions beyond the snowline, but the exact radial and vertical extension depend on disk mass, geometry, and stellar ultra-violet irradiation.Aims. The goal of this work is to present a computationally efficient method to compute ice and bare-grain opacities in protoplanetary disk models consistently with the chemistry and to investigate the effect of ice opacities on the physico-chemical state and optical appearance of the disk.Methods. A matrix of Mie efficiencies is pre-calculated for different ice species and thicknesses, from which the position dependent opacities of icy grains are then interpolated. This is implemented in the PRODIMO code by a self-consistent solution of ice opacities and the local composition of ices, which are obtained from our chemical network.Results. Locally, the opacity can change significantly, for example, an increase by a factor of more than 200 in the midplane, especially at ultra-violet and optical wavelengths, due to ice formation. This is mainly due to changes in the size distribution of dust grains resulting from ice formation. However, since the opacity only changes in the optically thick regions of the disk, the thermal disk structure does not change significantly. For the same reason, the spectral energy distributions (SEDs) computed with our disk models with ice opacities generally show only faint ice emission features at far-IR wavelengths. The ice absorption features are only seen in the edge-on orientation. The assumption made on how the ice is distributed across the grain size distribution (ice power law) influences the far-infrared and millimeter slope of the SED. The ice features and their strengths are influenced by the ice power law and the type of chemistry. Our models predict stronger ice features for observations that can spatially resolve the disk, particularly in absorption.Astrodynamics & Space Mission