Are These Planets or Brown Dwarfs? Broadly Solar Compositions from High-resolution Atmospheric Retrievals of ∼10-30 <i>M</i>_Jup Companions

Using Keck Planet Imager and Characterizer high-resolution (R similar to 35,000) spectroscopy from 2.29 to 2.49 mu m, we present uniform atmospheric retrievals for eight young substellar companions with masses of similar to 10-30 M-Jup, orbital separations spanning similar to 50-360 au, and T-eff between similar to 1500 and 2600 K. We find that all companions have solar C/O ratios and metallicities to within the 1 sigma-2 sigma level, with the measurements clustered around solar composition. Stars in the same stellar associations as our systems have near-solar abundances, so these results indicate that this population of companions is consistent with formation via direct gravitational collapse. Alternatively, core accretion outside the CO snowline would be compatible with our measurements, though the high mass ratios of most systems would require rapid core assembly and gas accretion in massive disks. On a population level, our findings can be contrasted with abundance measurements for directly imaged planets with m < 10 M-Jup, which show tentative atmospheric metal enrichment compared to their host stars. In addition, the atmospheric compositions of our sample of companions are distinct from those of hot Jupiters, which most likely form via core accretion. For two companions with T-eff similar to 1700-2000 K (kappa And b and GSC 6214-210 b), our best-fit models prefer a nongray cloud model with >3 sigma significance. The cloudy models yield 2 sigma-3 sigma lower T-eff for these companions, though the C/O and [C/H] still agree between cloudy and clear models at the 1 sigma level. Finally, we constrain (CO)-C-12/(CO)-C-13 for three companions with the highest signal-to-noise ratio data (GQ Lup b, HIP 79098b, and DH Tau b) and report v sin i and radial velocities for all companions.