Surprise Nondetection of Upsilon Andromedae b with MIRC-X and MYSTIC at the CHARA Array

Ground-based long baseline interferometry is a powerful tool for characterizing exoplanets that are too close to their host star to be imaged with single-dish telescopes. The CHARA Array can resolve companions down to 0.5 mas, allowing us in principle to directly measure the near-infrared spectra of nontransiting “hot Jupiter” exoplanets. We present data taken with the Michigan InfraRed Combiner-Exeter (MIRC-X) and MYSTIC instruments at the CHARA Array on the hot Jupiter Upsilon Andromedae b. By resolving the star–planet system, we attempt to directly detect the flux from the planet. We describe our self-calibration methods for modeling systematics in the closure phase data, which allows us to reach subdegree precision. Through combining multiple nights of data across two MIRC-X runs in 2019 and 2021, we achieved a very tentative detection of Ups And b in the H band at a planet/star contrast of 2–3 × 10−4. Unfortunately, we cannot confirm this detection with 2021 MYSTIC data in the K band, or in a 2023 joint MIRC-X and MYSTIC data set. We run updated global circulation models and create post-processed spectra for this planet, and report the resulting model spectra in H- and K bands as a function of orbital phase. We then run planetary injection tests to explore H/K-band contrast limits, and find that we can confidently recover planets down to a planet/star contrast of 1–2 × 10−4. We show that we are probing contrasts fainter than predicted by the model, making our nondetection surprising. We discuss prospects for the future in using this method to characterize companions with interferometry.