2025AJ....169..101B

Measuring the abundances of carbon- and oxygen-bearing molecules has been a primary focus in studying the atmospheres of hot Jupiters, as doing so can help constrain the carbon-to-oxygen (C/O) ratio. The C/O ratio can help reveal the evolution and formation pathways of hot Jupiters and provide a strong understanding of the atmospheric composition. In the last decade, high-resolution spectral analyses have become increasingly useful in measuring precise abundances of several carbon- and oxygen-bearing molecules. This allows for a more precise constraint of the C/O ratio. We present four transits of the hot Jupiter WASP-43b observed between 1.45 and 2.45 μm with the high-resolution Immersion GRating InfraRed Spectrometer on the Gemini-S telescope. We detected H₂O at a signal-to-noise ratio of 3.51. We tested for the presence of CH₄, CO, and CO₂, but we did not detect these carbon-bearing species. We ran a retrieval for all four molecules and obtained a water abundance of ${\mathrm{log}}_{10}({\rm{H}\,}_{2}\,\rm{O})=-2.2{4}_{-0.48}^{+0.57}$. We obtained an upper limit on the C/O ratio of C/O < 0.95. These findings are consistent with previous observations from the Hubble Space Telescope and the James Webb Space Telescope.