Are These Planets or Brown Dwarfs? Broadly Solar Compositions from High-Resolution Atmospheric Retrievals of 10-30 M_Jup Companions

Using Keck Planet Imager and Characterizer (KPIC) high-resolution (R 35000) spectroscopy from 2.29-2.49 μm, we present uniform atmospheric retrievals for eight young substellar companions with masses of 10-30 M_Jup, orbital separations spanning 50-360 au, and T_eff between 1500-2600 K. We find that all companions have solar C/O ratios, and metallicities, to within the 1-2σ 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. 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 1700-2000 K (kap And b and GSC 6214-210 b), our best-fit models prefer a non-gray cloud model with >3σ significance. The cloudy models yield 2-3σ lower T_eff for these companions, though the C/O and [C/H] still agree between cloudy and clear models at the 1σ level. Finally, we constrain 12CO/13CO for three companions with the highest S/N data (GQ Lup b, HIP 79098 b, and DH Tau b), and report vsin(i) and radial velocities for all companions.