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

TL;DR

This study used high-resolution spectroscopy with KPIC to detect atmospheric molecules in HR 8799 planets, developed a joint spectral fitting framework, and measured their rotational velocities, revealing potential mass-rotation correlations.

Contribution

It introduces a novel forward modeling approach for joint spectral and starlight fitting of directly imaged exoplanets at high resolution, enabling detailed atmospheric and rotational characterization.

Findings

Detected water and CO in HR 8799 c, d, e atmospheres.

Measured rotational velocities of HR 8799 d and e.

Found tentative evidence of anti-correlation between rotation speed and planet mass.

Abstract

Using the Keck Planet Imager and Characterizer (KPIC), we obtained high-resolution (R$\sim$35,000) $K$-band spectra of the four planets orbiting HR 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. These are the most challenging directly imaged exoplanets that have been observed at high spectral resolution to date when considering both their angular separations and flux ratios. We developed a forward modeling framework that allows us to jointly fit the spectra of the planets and the diffracted starlight simultaneously in a likelihood-based approach and obtained posterior probabilities on their effective temperatures, surface gravities, radial velocities, 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 HR 8799 e, and placed an upper limit of $< 14$~km/s of HR 8799 c. Under two different assumptions of their obliquities, we found tentative evidence that rotation velocity is anti-correlated with companion mass, which could indicate that magnetic braking with a circumplanetary disk at early times is less efficient at spinning down lower mass planets.