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Arxiv: MINDS. Hydrocarbons detected by JWST/MIRI in the inner disk of Sz28 consistent with a high C/O gas-phase chemistry Published: 7/19/2024 5:45:44 PM Updated: 7/19/2024 5:45:44 PM

Paper abstract: With the advent of JWST, we acquire unprecedented insights into the physicaland chemical structure of the inner regions of planet-forming disks whereterrestrial planet formation occurs. The very low-mass stars (VLMS) are knownto have a high occurrence rate of the terrestrial planets around them.Exploring the chemical composition of the gas in these inner regions of thedisks can aid a better understanding of the connection between planet-formingdisks and planets. The MIRI mid-Infrared Disk Survey (MINDS) project is a largeJWST Guaranteed Time program to characterize the chemistry and physical stateof planet-forming and debris disks. We use the JWST-MIRI/MRS spectrum toinvestigate the gas and dust composition of the planet-forming disk around thevery low-mass star Sz28 (M5.5, 0.12\,M_{\odot}). We use the dust-fitting tool(DuCK) to determine the dust continuum and to get constraints on the dustcomposition and grain sizes. We use 0D slab models to identify and fit themolecular spectral features, yielding estimates on the temperature, columndensity and the emitting area. To test our understanding of the chemistry inthe disks around VLMS, we employ the thermo-chemical disk model {P{\tinyRO}D{\tiny I}M{\tiny O}} and investigate the reservoirs of the detectedhydrocarbons. We explore how the C/O ratio affects the inner disk chemistry.JWST reveals a plethora of hydrocarbons, including \ce{CH3}, \ce{CH4},\ce{C2H2}, \ce{^{13}CCH2}, \ce{C2H6}, \ce{C3H4}, \ce{C4H2} and \ce{C6H6}suggesting a disk with a gaseous C/O\,>\,1. Additionally, we detect \ce{CO2},\ce{^{13}CO2}, \ce{HCN}, and \ce{HC3N}. \ce{H2O} and OH are absent in thespectrum. We do not detect PAHs. Photospheric stellar absorption lines of\ce{H2O} and \ce{CO} are identified. Notably, our radiation thermo-chemicaldisk models are able to produce these detected hydrocarbons in the surfacelayers of the disk when the ...