Titanium cold-trapping, numerous metals and ions, and the first ever unambiguous detection of vanadium oxide revealed on a ultra-hot giant exoplanet

Stefan Pelletier ( Université de Montréal )

Orbiting extremely close to their host stars and blasted by enormous amounts of radiation, ultra-hot Jupiters are giant gas planets that are home to some of the most extreme known atmospheric conditions. With orbital periods of a few days at most, these exoplanet are tidally-locked (like the moon around the Earth), causing a permanent hot dayside facing towards, and a cooler nightside facing away from their host star. This makes ultra-hot Jupiters unique astrophysical laboratories that present a tremendous diagnostic tool to probe the chemical contrasts governing their opposing, potentially drastically different hemispheres. The ultra-hot Jupiter WASP-76b took the exoplanet community by storm at the turn of the decade with evidence of iron condensation occurring from its hot dayside to its colder nightside (literally iron raining down). This landmark discovery was inferred from a distinct asymmetry in the absorption signal of gaseous iron evolving throughout the transit and introduced a new realm of opportunities for studying the day-to-night dichotomy of exoplanet atmospheres. I will present the results of follow-up transit observations of this fascinating planet using the new ultra-stable high-resolution MAROON-X spectrograph operating on Gemini-North. Our analysis of this data shows a plethora of gaseous metals and ions detected in WASP-76b’s transmission spectrum, a multitude of which also show similarly distinct asymmetric absorption signals throughout the transits. We also report the first ever unambiguous detection of vanadium oxide (VO) on an exoplanet (confirmed with 2 different instruments). VO is a strong UV light absorber that has long been thought to be a driver for thermal inversions in ultra-hot Jupiter atmospheres, but has historically been notoriously difficult to detect. We will also present an unprecedentedly thorough analysis of WASP-76b’s composition, precisely constraining the relative abundance of 15+ species in its atmosphere using a Bayesian high-resolution retrieval framework. We find that, while most elements are in agreement with equilibrium chemistry and a solar-like composition, a few species show clear deviations from model predictions. In particular, we measure V + VO abundances perfectly in line with expectations, but Ti + TiO abundances that are depleted by more than a factor of 100. This is a clear indication that titanium is missing from WASP-76b’s upper atmosphere, likely because it is cold-trapped on the nightside due to its lower condensation temperature relative to vanadium.