kappa And b is a fast rotator from KPIC High Resolution Spectroscopy

TL;DR

This study used high-resolution spectroscopy to analyze the planetary-mass companion kappa Andromedae b, revealing its rapid rotation, atmospheric composition, and updated orbital parameters, advancing understanding of exoplanet spin and atmospheric properties.

Contribution

First high-resolution spectral analysis of kappa Andromedae b, measuring its rapid rotation, atmospheric parameters, and refining its orbital characteristics.

Findings

Measured a high rotation velocity of 38.42 km/s, near 50% of breakup velocity.

Detected water and CO in the atmosphere via cross correlation.

Updated the orbital fit with new radial velocity measurement.

Abstract

We used the Keck Planet Imager and Characterizer (KPIC) to obtain high-resolution (R$\sim$35,000) K-band spectra of kappa Andromedae b, a planetary-mass companion orbiting the B9V star, kappa Andromedae A. We characterized its spin, radial velocity, and bulk atmospheric parameters through use of a forward modeling framework to jointly fit planetary spectra and residual starlight speckles, obtaining likelihood-based posterior probabilities. We also detected H$_{2}$O and CO in its atmosphere via cross correlation. We measured a $v\sin(i)$ value for kappa And b of $38.42\pm{0.05}$ km/s, 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, which we use with existing astrometry and RV measurements to update the orbital fit. We also measure an effective temperature of $1700\pm{100}$ K and a $\log(g)$ of $4.7\pm{0.5}$ cgs dex.