Into the Darkness: CO detection on the Night-side of the Highly Irradiated WASP-33b

Georgia Mraz ( Université McGill )

We have obtained observations of the ultra hot Jupiter WASP-33b with the Spectro-Polarimetre Infra-Rouge (SPIRou). WASP-33b is amongst the most highly irradiated exoplanets with a period of 1.2 days and a substellar equilibrium temperature of ~3900K. The day-side of WASP-33b was recently observed with the same instrument and shows evidence of CO and Fe [Darveau-Bernier, 2024 in prep]. Subsequently, we measured the night-side thermal emission over five nights to search for signatures of CO. Three pre-transit observations and two post transit observation allowed sensitivity to either the western or eastern terminator. From retrievals, we confirm a detection of CO in absorption after transit giving the first detection of night-side planetary emission as high resolution. The CO in absorption suggests that the night- side doesn’t have the temperature inversion seen on the day-side; pointing to the day-side inversion being driven by stellar radiation. Likewise, the non- detection of thermal emission from the western terminator but presence on the eastern is consistent with heat advection by an eastward jet. Complementing the day-side observations, this night-side detection of CO provides evidence of the expected temperature profile. I will present this state of the art night-side emission detection from the ground via high spectral resolution. This method could offer an economical way to constrain the vertical temperature profile of exoplanet night-sides, compared to space- based spectroscopy.