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Arxiv: High-resolution Elemental Abundance Measurements of Cool JWST Planet Hosts Using AutoSpecFit: An Application to the Sub-Neptune K2-18b's Host M dwarf Published: 7/10/2024 8:33:32 PM Updated: 7/10/2024 8:33:32 PM

Paper abstract: We present an in-depth, high-resolution spectroscopic analysis of the M dwarfK2-18 that hosts a sub-Neptune exoplanet in its habitable zone. We show ourtechnique to accurately normalize the observed spectrum, which is crucial for aproper spectral fitting. We also introduce a new automatic, line-by-linemodel-fitting code, AutoSpecFit, that performs an iterative {\chi}^{2}minimization process to measure individual elemental abundances of cool dwarfs.We apply this code to the star K2-18, and measure the abundance of 10 elements- C, O, Na, Mg, Al, K, Ca, Sc, Ti, and Fe. We find these abundances moderatelysupersolar, except for Fe with a slightly subsolar abundance. The accuracy ofthe inferred abundances is limited by the systematic errors due to uncertainstellar parameters. We also derive the abundance ratios associated with severalplanet-building elements such as Al/Mg, Ca/Mg, Fe/Mg, and (a solar-like)C/O=0.568 \pm 0.026, which can be used to constrain the chemical compositionand the formation location of the exoplanet. On the other hand, the planetK2-18 b has attracted considerable interest, given the JWST measurements of itsatmospheric composition. Early JWST studies reveal an unusual chemistry for theatmosphere of this planet, which is unlikely to be driven by formation in adisk of unusual composition. The comparison between the chemical abundances ofK2-18 b from future JWST analyses and those of the host star can providefundamental insights into the formation of this planetary system.