κ Andromedae b Is a Fast Rotator from KPIC High-resolution Spectroscopy

We used the Keck Planet Imager and Characterizer to obtain high-resolution ( R ∼ 35,000) K -band spectra of κ Andromedae b, a planetary-mass companion orbiting the B9V star, κ Andromedae A. We characterized its spin, radial velocity, and bulk atmospheric parameters through use of a forward-modeling framework to jointly fit planetary spectra and residual starlight speckles, obtaining likelihood-based posterior probabilities. We also detected H _2 O and CO in its atmosphere via cross correlation. We measured a $v\sin (i)$ value for κ Andromedae b of 38.42 ± 0.05 km s ^−1 , allowing us to extend our understanding of the population of close-in bound companions at higher rotation rates. This rotation rate is one of the highest spins relative to breakup velocity measured to date, at close to 50% of breakup velocity. We identify a radial velocity $-{17.35}_{-0.09}^{+0.05}$ km s ^−1 , which we use with existing astrometry and radial velocity measurements to update the orbital fit. We also measure an effective temperature of 1700 ± 100 K and a $\mathrm{log}(g)$ of 4.7 ± 0.5 cgs dex.