Retrieving C and O Abundance of HR 8799 c by Combining High- and Low-Resolution Data

TL;DR

This study combines high- and low-resolution spectroscopic data to accurately determine the atmospheric chemical abundances of HR 8799 c, shedding light on its formation history and atmospheric composition.

Contribution

It introduces a novel atmospheric retrieval method that integrates diverse spectral data to constrain C and O abundances in exoplanet atmospheres.

Findings

HR 8799 c has super-stellar C and O abundances.

The planet's C/O ratio is approximately solar.

Results suggest early formation with subsequent atmospheric enrichment.

Abstract

The formation and evolution pathway for the directly-imaged multi-planetary system HR 8799 remains mysterious. Accurate constraints on the chemical composition of the planetary atmosphere(s) are key to solving the mystery. We perform a detailed atmospheric retrieval on HR 8799~c to infer the chemical abundances and abundance ratios using a combination of photometric data along with low- and high-resolution spectroscopic data (R$\sim$20-35,000). We specifically retrieve [C/H], [O/H], and C/O and find them to be 0.55$^{+0.36}_{-0.39}$, 0.47$^{+0.31}_{-0.32}$, and 0.67$^{+0.12}_{-0.15}$ at 68\% confidence. The super-stellar C and O abundances, yet a stellar C/O ratio, reveal a potential formation pathway for HR 8799~c. Planet c, and likely the other gas giant planets in the system, formed early on (likely within $\sim$1 Myr), followed by further atmospheric enrichment in C and O through the accretion of solids beyond the CO iceline. The enrichment either preceded or took place during the early phase of the inward migration to the planet current locations.