  Young, self-luminous super-Jovian companions discovered by direct imaging
provide a challenging test of 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 MJup 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 H2O 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 (vsini),
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{\sigma} confidence
intervals), values consistent with those of the Sun and with a companion
formation via gravitational instability or core accretion. The projected
rotation velocity < 7.8 km/s is small relative to other directly imaged
companions with similar masses and ages. This may imply a near 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.
