The water is strong with this one: the emission features of Kelt-20b

Title: Strong H2O and CO emission features in the spectrum of KELT-20b driven by stellar UV irradiation

Authors: Guangwei Fu, David K. Sing, Joshua D. Lothringer, Drake Deming, Jegug Ih, Eliza Kempton, Matej Malik, Thaddeus D. Komacek, Megan Mansfield, Jacob L. Bean

First Author’s Institution: University of Maryland

Status: Accepted for publication in ApJL, available on arxiv.

There are several factors that determine what an exoplanet’s atmosphere is made of. Hot Jupiters, which are gas giant planets that orbit close to their stars (with a period < 10 days!), have atmospheres  whose composition can be determined by the intense radiation they receive from their host. They can have water in their atmospheres as long as their temperature is not too high: if it is above 2800 K, the water dissociates. So, if the planet is too hot, the spectral emission features of water become harder to detect, since there is less water due to this dissociation. On the other hand, if the host star has a high temperature, the water spectral emission features become easier to detect. This is because the metal oxides in these exoplanets absorb more radiation from the star, which heats up the planet’s atmosphere and leads to a phenomenon called thermal inversion, where the upper layers of the atmosphere become hotter than the lower layers. So, a planet that orbits a hot star (such as an A-type star) and has low dayside temperatures should have strong emission features for molecules such as water and carbon monoxide.

The authors of today’s paper look at the emission features of Kelt-20b (an ultra-hot Jupiter that orbits at only 0.0542 AU from its host star) using spectral data from TESS, Hubble Space Telescope’s WFC3/G141 and Spitzer. They find that Kelt-20b has strong emission features for water (H2O) and carbon monoxide (CO), likely due to the combination of a hot, A-type host star, and a low dayside atmospheric temperature for the planet (T < 2800 K) which leads to a strong thermal inversion on the planet’s atmosphere.

Obtaining Kelt-20b’s Spectral Data 

The emission spectrum of Kelt-20b was obtained using transit spectroscopy. Using the combined data from three instruments, the authors analyze Kelt-20b’s spectrum on the 0.6 – 4.5 µm wavelength range. The emission spectrum of Kelt-20b is presented on Figure 1, along with atmospheric models (one retrieval model, which fits the best atmospheric model for your data, and one forward model, which computes atmospheric models from initial parameters and analyzes which of the parameters best fit your data) that determine whether the spectrum is self-consistent and makes physical sense. In figure 1, the models also show the T-P (temperature-pressure) profile of Kelt-20b. The thermal inversion feature would yield a higher temperature at smaller pressures (so in the upper layers of the atmosphere). 

Figure 1. Left Plot: The Kelt-20b spectrum is presented in black. The ATMO retrieval model (purple) and Phoenix forward model (pink) show whether the data is physically consistent with current theoretical models for hot Jupiter atmospheres. The gray line is a blackbody model at 2402 K. The blue shaded region represents the 1-\sigma uncertainties on the ATMO model. The water emission feature is detected at 1.4 µm and the CO emission line is detected at 4.5 µm, which can be noted by the excess between the gray line and the data/model lines on these wavelengths. Right Plot: The T-P profile of Kelt-20b is presented along the models. The thermal inversion profile of higher temperatures at lower pressures can be identified in both models.

Comparing Kelt-20b’s emission spectrum to other Hot Jupiters’

The authors also compared the water and CO feature strength of Kelt-20b with that of other hot Jupiters. In order to make these comparisons, they use the “water feature strength metric”, called SH2O, which is a metric that compares the water part of the spectrum to its surrounding ranges. If there is a water absorption feature, SH2O is expected to be positive and if there is a water emission feature, SH2O is expected to be negative. Kelt-20b’s SH2O is found to be -0.097± 0.02, which is a very large water emission feature. 

Figure 2. The metrics of Kelt-20b are compared to models of hot Jupiters with dayside temperatures of 2677 K, 3184 K and 4500 K. Top panel: the water feature strength is plotted as a function of host star temperature. Kelt-20 b has a high host star temperature and a low atmospheric daytime temperature, which plces it in a unique spot when compared to other hot Jupiters. Bottom panel: Kelt-20b has a big difference between the blackbody temperature measured by WFC3/G141 and the Spitzer 4.5 µm band, which indicates a strong CO emission feature when compared to other hot Jupiters.

The CO comparison was made using the detection differences between the WFC3/G141 and the Spitzer 4.5 µm bands: for an inverted TP profile, Spitzer looks higher up in the atmosphere than Hubble’s WFC3/G141, so a bigger difference in the two values would indicate a stronger thermal inversion and therefore a stronger CO emission feature. The results of these comparisons can be found on Figure 2.

 This work shows us that the host star can have a big impact on an exoplanet’s atmospheric emission features. Knowing the host star of exoplanets adds a new parameter to help us understand exoplanet atmospheres and their properties in the future!

Astrobite edited by: William Balmer

About Clarissa Do O

I am a second year physics graduate student at UC San Diego. I study exoplanet orbital dynamics and also work on exoplanet instrumentation. My current work is on the adaptive optics upgrade of the Gemini Planet Imager 2.0, an instrument that aims to directly image and characterize exoplanets.

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