A solar C/O and sub-solar metallicity in a hot Jupiter atmosphere

Abstract

The origins of gas giant planets orbiting close to their host stars ("hot Jupiters") remain a mystery despite more than a quarter-century of study 1 . The atmospheric compositions of these planets are highly sought after to provide insight to their formation location in protoplanetary disks, how they migrated to be so close to their host stars, and the relative role of solid versus gas accretion during their assembly 2 . However, simultaneous, bounded constraints on both carbon and oxygen abundances, which are key for understanding giant planet formation 3,4,5,6 , have been elusive 7,8,9 . Here, we report precise abundance measurements of both water and carbon monoxide in a hot Jupiter atmosphere via ground-based, high resolution spectroscopy. From these constraints on the primary carbon-and oxygenbearing molecules, paired with upper limits on other minor volatile elemental carriers, we are able to derive the atmospheric elemental metal enrichment (metallicity) and the carbonto-oxygen ratio (C/O). The inferred atmospheric metallicity is slightly sub-stellar (-0.48 +0.15 −0.13

Evidence that the Hot Jupiter WASP-77 A b Formed Beyond Its Parent Protoplanetary Disk's H2O Ice Line

“…We find that carbon and oxygen are 3.6 times and 5.2 times less abundant in the atmosphere of WASP-77 A b than in the photosphere of WASP-77 A. Our abundance inferences for WASP-77 A increase the statistical significance of the Line et al (2021) observation that WASP-77 A b's atmosphere has sub-stellar carbon and oxygen abundances, from 2.6 σ to 2.9 σ for carbon and from 3.4 σ to 5.1 σ for oxygen. Our C/O abundance ra- Amarsi et al (2020) for the three lines of the oxygen triplet are limited to a mean correction.…”

“…In this article, we infer photospheric and fundamental stellar parameters as well as individual elemental abundances-including carbon and oxygen-for the hot Jupiter host star WASP-77 A. Atmospheric carbon and oxygen abundances for the hot Jupiter WASP-77 A b were recently published by Line et al (2021), and the carbon and oxygen abundances we infer for WASP-77 A qualitatively change the interpretation of WASP-77 A b's atmospheric abundances. We describe in Section 2 the high-resolution optical spectrum we collected for WASP-77 A.…”

Relative abundance constraints from high-resolution optical transmission spectroscopy of WASP-121b, and a fast model-filtering technique for accelerating retrievals

“…Birkby et al 2017;Esteves et al 2017;Nugroho et al 2021); however, more recent studies performing retrievals have reverted to basismodel fitting or PCA which can be applied more quickly within each likelihood calculation (e.g. Brogi & Line 2019;Pelletier et al 2021;Line et al 2021), whereas S R is an iterative process and is much too slow. We later introduce in Sect.…”

“…Note that it is possible to partition the Brogi & Line (2019) log-likelihood into separate spectra, which in principle takes into account the time-(and order-) dependence of the noise (e.g. Line et al 2021), but it is difficult to account for the wavelength dependence in this way. It would similarly be possible to partition Eq.…”

“…One proposed solution is to inject the forward model atmosphere into a representation of the telluric and stellar model, and re-process the forward model in the same way as the data. Indeed, this was demonstrated to work successfully both with airmass de-trending and also with PCA filtering (Brogi & Line 2019;Pelletier et al 2021;Line et al 2021). However, this injection and re-filtering process is generally slow, and in many cases can become the bottleneck in each likelihood calculation.…”