Ionized calcium in the atmospheres of two ultra-hot exoplanets WASP-33b and KELT-9b

TL;DR

This study detects ionized calcium in the upper atmospheres of the ultra-hot exoplanets WASP-33b and KELT-9b, revealing high-altitude layers and suggesting hydrodynamic outflows influence their atmospheric composition.

Contribution

First detection of ionized calcium in the atmospheres of the hottest known giant exoplanets, using high-resolution transit spectroscopy.

Findings

Ca II lines are deeper than hydrostatic model predictions.

Calcium is predominantly ionized in the upper atmospheres.

High-altitude layers are close to planetary Roche lobes.

Abstract

Ultra-hot Jupiters are emerging as a new class of exoplanets. Studying their chemical compositions and temperature structures will improve the understanding of their mass loss rate as well as their formation and evolution. We present the detection of ionized calcium in the two hottest giant exoplanets - KELT-9b and WASP-33b. By utilizing transit datasets from CARMENES and HARPS-N observations, we achieved high confidence level detections of Ca II using the cross-correlation method. We further obtain the transmission spectra around the individual lines of the Ca II H&K doublet and the near-infrared triplet, and measure their line profiles. The Ca II H&K lines have an average line depth of 2.02 $\pm$ 0.17 % (effective radius of 1.56 Rp) for WASP-33b and an average line depth of 0.78 $\pm$ 0.04 % (effective radius of 1.47 Rp) for KELT-9b, which indicates that the absorptions are from very high upper atmosphere layers close to the planetary Roche lobes. The observed Ca II lines are significantly deeper than the predicted values from the hydrostatic models. Such a discrepancy is probably a result of hydrodynamic outflow that transports a significant amount of Ca II into the upper atmosphere. The prominent Ca II detection with the lack of significant Ca I detection implies that calcium is mostly ionized in the upper atmospheres of the two planets.