Are these planets or brown dwarfs? Broadly solar compositions from high-resolution atmospheric retrievals of ~10-30 MJup companions

Using Keck Planet Imager and Characterizer (KPIC) high-resolution (R~35000) spectroscopy from 2. 29-2. 49 m, we present uniform atmospheric retrievals for eight young substellar companions with masses of ~10-30 MJup, orbital separations spanning ~50-360 au, and Tₑff between ~1500-2600 K. We find that all companions have solar C/O ratios, and metallicities, to within the 1-2 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 m3 significance. The cloudy models yield 2-3 lower Tₑff for these companions, though the C/O and C/H still agree between cloudy and clear models at the 1 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.