Atmospheric Characterization of the Super-Jupiter HIP 99770 b with KPIC

Abstract Young, self-luminous super-Jovian companions discovered by direct imaging provide a challenging test for planet formation and evolution theories. By spectroscopically characterizing the atmospheric compositions of these super-Jupiters, we can constrain their formation histories. Here we present studies of the recently discovered HIP 99770 b, a 16 M Jup high-contrast companion on a 17 au orbit, using the fiber-fed high-resolution spectrograph KPIC ( R ∼ 35,000) on the Keck II telescope. Our K -band observations led to detections of H 2 O and CO in the atmosphere of HIP 99770 b. We carried out free retrieval analyses using petitRADTRANS to measure its chemical abundances, including the metallicity and C/O ratio, projected rotation velocity ( v sin i ), and radial velocity (RV). We found that the companion’s atmosphere has C/O = 0.55 − 0.04 + 0.06 and M/H = 0.26 − 0.23 + 0.24 (1 σ confidence intervals), values consistent with those of the Sun and with a companion formation via gravitational instability or core accretion. The projected rotation velocity v sin ( i ) 7.8 km s −1 is small relative to other directly imaged companions with similar masses and ages. This may imply a nearly pole-on orientation or effective magnetic braking by a circumplanetary disk. In addition, we added the companion-to-primary relative RV measurement to the orbital fitting and obtained updated constraints on orbital parameters. Detailed characterization of super-Jovian companions within 20 au like HIP 99770 b is critical for understanding the formation histories of this population.