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Arxiv: Toward Exoplanet Transit Spectroscopy Using JWST/MIRI's Medium Resolution Spectrometer Published: 7/22/2024 9:00:04 PM Updated: 7/22/2024 9:00:04 PM

Paper abstract: The Mid-Infrared Instrument (MIRI)'s Medium Resolution Spectrometer (the MRS)on JWST has potentially important advantages for transit and eclipsespectroscopy of exoplanets, including lack of saturation for bright host stars,wavelength span to longward of 20 microns, and JWST's highest spectralresolving power. We here test the performance of the MRS for time seriesspectroscopy by observing the secondary eclipse of the bright stellar eclipsingbinary R Canis Majoris. Our observations push the MRS into saturation at theshortest wavelength, more than for any currently known exoplanet system. Wefind strong charge migration between pixels that we mitigate using a customdata analysis pipeline. Our data analysis recovers much of the spatial chargemigration by combining detector pixels at the group level, via weighting by thepoint spread function. We achieve nearly photon-limited performance in timeseries data at wavelengths longward of 5.2 microns. In 2017, Snellen et al.suggested that the MRS could be used to detect carbon dioxide absorption fromthe atmosphere of the temperate planet orbiting Proxima Centauri. We infer thatthe relative spectral response of the MRS versus wavelength is sufficientlystable to make that detection feasible. As regards the secondary eclipse ofthis Algol-type binary, we measure the eclipse depth by summing our spectraover the wavelengths in four channels, and also measuring the eclipse depth asobserved by TESS. Those eclipse depths require a temperature for the secondarystar that is significantly hotter than previous observations in the optical tonear-IR, probably due to irradiation by the primary star. At full spectralresolution of the MRS, we find atomic hydrogen recombination emission lines inthe secondary star, from principal quantum levels n = 7, 8, 10, and 14.