A temperature inversion with atomic iron in the ultra-hot dayside atmosphere of WASP-189b

TL;DR

This study detects atomic iron in the atmosphere of ultra-hot Jupiter WASP-189b, providing direct evidence of a temperature inversion caused by Fe I absorption, using high-resolution spectral observations and modeling.

Contribution

First detection of atomic iron causing a temperature inversion in an ultra-hot Jupiter's atmosphere through high-resolution spectroscopy.

Findings

Fe I detected in emission indicating a temperature inversion

Temperature at the inversion top is about 4320 K, hotter than equilibrium temperature

Strong temperature inversion likely caused by Fe I absorption

Abstract

Temperature inversion layers are predicted to be present in ultra-hot giant planet atmospheres. Although such inversion layers have recently been observed in several ultra-hot Jupiters, the chemical species responsible for creating the inversion remain unidentified. Here, we present observations of the thermal emission spectrum of an ultra-hot Jupiter, WASP-189b, at high spectral resolution using the HARPS-N spectrograph. Using the cross-correlation technique, we detect a strong Fe I signal. The detected Fe I spectral lines are found in emission, which is direct evidence of a temperature inversion in the planetary atmosphere. We further performed a retrieval on the observed spectrum using a forward model with an MCMC approach. When assuming a solar metallicity, the best-fit result returns a temperature of $4320_{-100}^{+120}$ K at the top of the inversion, which is significantly hotter than the planetary equilibrium temperature (2641 K). The temperature at the bottom of the inversion is determined as $2200_{-800}^{+1000}$ K. Such a strong temperature inversion is probably created by the absorption of atomic species like Fe I.