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

TL;DR

This study measures elemental abundances, especially C/O ratios, in host stars of directly imaged companions to understand planet formation, revealing diversity in stellar compositions and implications for planetary atmospheres.

Contribution

It provides high-resolution spectroscopic measurements of 16 elements in five host stars, highlighting the diversity in C/O ratios and their significance for planet formation theories.

Findings

Solar C/O ratios in three stars, sub-solar in two.

Detection of sub-solar C/O ratios at 4σ significance for PZ Tel and β Pic.

Population-level analysis shows super-solar C and O abundances.

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 $\beta$ Pic (0.22 $\pm$ 0.06). The $4\sigma$ sub-solar C/O detections for PZ Tel and $\beta$ 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