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Using the Keck Planet Imager and Characterizer (KPIC), we obtainedhigh-resolution (R$\sim$35,000) $K$-band spectra of the four planets orbitingHR 8799. We clearly detected \water{} and CO in the atmospheres of HR 8799 c,d, and e, and tentatively detected a combination of CO and \water{} in b. Theseare the most challenging directly imaged exoplanets that have been observed athigh spectral resolution to date when considering both their angularseparations and flux ratios. We developed a forward modeling framework thatallows us to jointly fit the spectra of the planets and the diffractedstarlight simultaneously in a likelihood-based approach and obtained posteriorprobabilities on their effective temperatures, surface gravities, radialvelocities, and spins. We measured $v\sin(i)$ values of$10.1^{+2.8}_{-2.7}$~km/s for HR 8799 d and $15.0^{+2.3}_{-2.6}$~km/s for HR8799 e, and placed an upper limit of $< 14$~km/s of HR 8799 c. Under twodifferent assumptions of their obliquities, we found tentative evidence thatrotation velocity is anti-correlated with companion mass, which could indicatethat magnetic braking with a circumplanetary disk at early times is lessefficient at spinning down lower mass planets.

Detection and Bulk Properties of the HR 8799 Planets with High Resolution Spectroscopy