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 highly irradiated with a period of 1.2 days and a substellar equilibrium temperature of ~3700K. The day- side of WASP-33b was recently observed with the same instrument and shows evidence of CO and Fe [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 observations allowed sensitivity to regions near the western or eastern terminators, respectively. 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, as we would expect for a day- side inversion driven by absorption of 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’ll present this 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, phase-resolved spectroscopy.