Using chemical kinetics models to unravel the origin of excess carbon and lack of nitrogen in planet-forming environments Javiera Diaz Berrios, Catherine Walsh University of Leeds, UK The material available in the planet-forming disks of dust and gas that surround young stars will determine the atmospheric composition of planets. The chemistry of these disks is diverse and is governed by gas-phase kinetics, surface reactions on dust grains, and the coupling between the two via adsorption and desorption. Thus, these disks have a rich chemistry, with more than 20 molecules detected in disks to date [1]. Because of the importance of the composition of these regions for planet formation, it is crucial to understand what processes and chemical reactions are occurring in these sources. To do this, it is necessary to constrain the abundance and distribution of organic volatiles in the planet-forming regions of disks formed from the main carriers of carbon, oxygen, nitrogen, and sulphur [2,3]. Previous observations of planet-forming disks around low-mass stars using the Spitzer Space Telescope hinted at a rich chemistry of small organic volatiles in the inner regions of the disks [4]. Motivated by these results, chemical kinetics models were developed to study the abundance and distribution of small organic molecules in disks around stars of different spectral type: M-dwarfs (Mstar=0.1Msun), T Tauri (0.5Msun) and Herbig Ae (2Msun) stars. These simulations predicted that the small organic molecules, C 2 H 2 and HCN, are more abundant in disks around low-mass stars than around higher-mass stars [5], in agreement with the observations available at that time. With the release of JWST (James Webb Space Telescope), observing the inner region of the planet-forming environment around young stars at high sensitivity and spectral resolution is now possible. JWST observations of the composition of the disk around the low-mass star J160532 (0.14Msun; [6]) revealed a planet-forming region rich in hydrocarbons (e.g., C 2 H 2 , C 4 H 2 , and C 6 H 6 ) and devoid of nitrogen carriers (e.g., NH 3 ), in contrast with predictions from chemical kinetics models [5]. The lack of NH3 in the inner regions of disks is a particular puzzle as it is predicted to be abundant in models [7]. To understand why the models are not fully consistent with observations, we employ chemical kinetics models using the physical structure of a planet-forming disk around an M-dwarf star and compute the abundances of key volatiles in the inner disk. We present results from models in which we test different radial and vertical distributions of elemental abundances (C, O, and N), thereby tuning the amount of carbon, oxygen, and nitrogen in the disk to better reproduce current observations. Processes in the disk, such as dust grain growth, settling, and drift, can drastically alter the elemental ratios in the gas and available for chemistry. We use the results of these models to determine the origin of the rich hydrocarbon chemistry and the corresponding lack of ammonia in the planet-forming regions around low-mass stars. References 1. McGuire+2022, ApJS, 259, 30
2. Pontoppidan+2014, PPVI 3. Oberg+2021, PhR, 893, 1 4. Pascucci,+2013, ApJ, 779, 178 5. Walsh+2015, A&A, 582, A88 6. Tabone+2023, NatAstron 7, 805–814 7. van Dishoeck+2023, FaradayDiscuss
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