KPF Confirms a Polar Orbit for KELT-18 b

TL;DR

This study reports spectroscopic transit observations of KELT-18 b, confirming its polar orbit and suggesting high-eccentricity migration possibly caused by a binary stellar companion, contributing to understanding hot Jupiter orbital dynamics.

Contribution

First spectroscopic transit analysis of KELT-18 b confirming its polar orbit and proposing a migration mechanism involving a binary companion.

Findings

KELT-18 b has a sky projected obliquity of -94.8 degrees.

The true 3D obliquity is approximately 92 degrees.

Evidence suggests high-eccentricity migration due to Kozai-Lidov oscillations.

Abstract

We present the first spectroscopic transit results from the newly commissioned Keck Planet Finder on the Keck-I telescope at W. M. Keck Observatory. We observed a transit of KELT-18 b, an inflated ultra-hot Jupiter orbiting a hot star ($T_\text{eff} = 6670$ K) with a binary stellar companion. By modeling the perturbation to the measured cross correlation functions using the Reloaded Rossiter-McLaughlin technique, we derived a sky projected obliquity of $\lambda = -94.8 \pm 0.7$ deg ($\psi = 93.8_{-1.8}^{+1.6}$ deg for isotropic $i_\star$). The data are consistent with an extreme stellar differential rotation ($\alpha = 0.9$), though a more likely explanation is moderate center-to-limb variations of the emergent stellar spectrum. We see additional evidence for the latter from line widths increasing towards the limb. Using loose constraints on the stellar rotation period from observed variability in the available TESS photometry, we were able to constrain the stellar inclination and thus the true 3D stellar obliquity to $\psi = 91.7_{-1.8}^{+2.2}$ deg. KELT-18 b could have obtained its polar orbit through high-eccentricity migration initiated by Kozai-Lidov oscillations induced by the binary stellar companion KELT-18 B, as the two likely have a large mutual inclination as evidenced by Gaia astrometry. KELT-18 b adds another data point to the growing population of close-in polar planets, particularly around hot stars.