A High-Resolution Spectroscopic Survey of Directly Imaged Companion Hosts: I. Determination of diagnostic stellar abundances for planet formation and composition

TL;DR

This paper reports the first results of an optical high-resolution spectroscopic survey of directly imaged planet host stars, aiming to determine metallicities and abundances for 15 elements and to derive key ratios such as C/O, C/S, and O/S to illuminate planet formation pathways. The analysis combines spectral modeling with equivalent width measurements, applied to five well-studied host stars to establish a robust abundance-determination pipeline. The results show solar C/O for four stars (HR 8799, 51 Eri, HD 984, GJ 504) and a super-solar C/O for HD 206893, with agreement between modeling and EW methods and no evidence for extreme metallicities; these findings imply that a metal-rich environment is not a strict prerequisite for forming wide-separation gas giants. The measured abundances, including Mg/Si and other ratios, provide additional constraints for planet formation models, and set the stage for JWST-driven sulfur abundance measurements in directly imaged planets.

Abstract

We present the first results of an extensive spectroscopic survey of directly imaged planet host stars. The goal of the survey is the measurement of stellar properties and abundances of 15 elements (including C, O, and S) in these stars. In this work, we present the analysis procedure and the results for an initial set of five host stars, including some very well-known systems. We obtain C/O ratios using a combination of spectral modeling and equivalent width measurements for all five stars. Our analysis indicates solar C/O ratios for HR 8799 (0.59 $\pm$ 0.11), 51 Eri (0.54 $\pm$ 0.14), HD 984 (0.63 $\pm$ 0.14), and GJ 504 (0.54 $\pm$ 0.14). However, we find a super-solar C/O (0.81 $\pm$ 0.14) for HD 206893 through spectral modeling. The ratios obtained using the equivalent width method agree with those obtained using spectral modeling but have higher uncertainties ($\sim$0.3 dex). We also calculate the C/S and O/S ratios, which will help us to better constrain planet formation, especially once planetary sulfur abundances are measured using JWST. Lastly, we find no evidence of highly elevated metallicities or abundances for any of our targets, suggesting that a super metal-rich environment is not a prerequisite for large, widely separated gas planet formation. The measurement of elemental abundances beyond carbon and oxygen also provides access to additional abundance ratios, such as Mg/Si, which could aid in further modeling of their giant companions.