A High-Resolution Spectroscopic Survey of Directly Imaged Companion Hosts: II. Diversity in C/O Ratios among Host Stars

TL;DR

The study tackles how stellar chemical compositions influence the interpretation of directly imaged exoplanets by providing high-resolution abundances for five southern host stars across 16 elements and the C/O ratio. Using spectral template fitting and equivalent width methods with GHOST data, the authors derive precise abundances and compare stellar C/O ratios to those of their companions, revealing both solar and sub-solar values and indicating super-solar volatile content in the broader population. They also perform population-level comparisons and a Kolmogorov–Smirnov test to assess differences between stellar populations, finding limited evidence to distinguish them based on C/O alone. The results underscore the importance of accurate stellar abundances for constraining planet formation scenarios and suggest diverse formation histories among directly imaged companions.

Abstract

The era of JWST has enabled measurements of abundances of elements such as C, O, and even Na, S, K, and Fe in planetary atmospheres to very high precisions ($\sim$0.1 dex). Accurate inference of planet formation using these elemental abundances require the corresponding abundance measurements for the host star. We present the second set of results from our high-resolution spectroscopic survey of directly imaged companion host stars, measuring abundances of 16 elements (including C, O, Na, Mg, Si, S, K and Fe) for five directly imaged companion host stars. Using both the spectral fitting and the equivalent width methods, we find solar C/O ratios for HR 2562 (0.58 $\pm$ 0.09), AB Pic (0.50 $\pm$ 0.14), and YSES 1 (0.45 $\pm$ 0.05), and sub-solar C/O ratios for PZ Tel (0.28 $\pm$ 0.05) and $β$ Pic (0.22 $\pm$ 0.06). The $4σ$ sub-solar C/O detections for PZ Tel and $β$ Pic highlight the importance of accurate stellar C/O estimates for constraining planet formation. Subsequently, we combine our abundances with those from our previous work to measure population-level average elemental abundances. We find super-solar carbon and oxygen for this stellar population, indicating that the protoplanetary disks around these stars were potentially rich in volatiles. We compare stellar C/O to those of their companions, revealing super-stellar C/O for several objects that suggest planet-like formation mechanisms. We also compare the C/O of our directly imaged companion host star population with other planet host stars using the Kolmogorov-Smirnov Test, which indicates insufficient evidence to differentiate between the various stellar populations