Giant exoplanets orbiting close to their host stars have high temperatures because they receive immense stellar irradiation. The extreme energy input leads to the expansion of the atmosphere and the escape of neutral hydrogen. KELT-9b is the hottest exoplanet known so far. It is particularly interesting to study its atmosphere as the planet receives immense stellar irradiation from its host A0-type star.

We have detected an extended hot hydrogen atmosphere around KELT-9b (Yan & Henning, Nature Astronomy, 2018). The detection was achieved by measuring the atomic hydrogen absorption during transit with the Balmer Hα line.

The observation was performed with CARMNESE —- a high resolution spectrograph (R ~ 94, 600) mounted on the 3.5m telescope of the Calar Alto Observatory. We observed two transits on August 6 and September 21, 2017. Data reduction has three main steps: (1) Removal of telluric lines by using a theoretical H2O model. (2) Removal of stellar lines by using the out-of-transit spectrum as template. (3) Rebin the two nights spectra with a 0.01 phase step. In this way, we get the Hα transmission spectrum as shown in the figure below.


The upper panel of the above figure shows the observed Hα transmission spectrum (ratios between the observed spectra and the reference out-of-transit spectrum). X-axis is wavelength expressed in RV relatively to the Hα line centre. The middle panel is the best-fit model from MCMC analysis. The model includes the Hα transmission spectrum and the stellar line profile change. The stellar line profile change is due to the Rossiter-McLaughlin effect and the Centre-to-Limb Variation effect. The blue dashed line indicates the RV of the planetary orbital motion. The green dashed line indicates the RV of the rotating stellar surface which is obscured by the planet.

We further obtained the Hα absorption line profile by combining all the in-transit spectra. The observed absorption has a line depth of 1.15% at the line center.

Such a strong absorption corresponds to an effective radius of 1.64 Rp. The Roche lobe of the planet is only ~1.91 Rp, because the planet is orbiting very close to the massive host star. Since Hα absorption is originated from excited atoms (n=2), the ground-state atoms are several magnitude more than excited atoms. Thus we infer the hydrogen envelope almost fills up the Roche lobe and the atmosphere is escaping!



Yan & Henning, Nature Astronomy (2018)

An extended hydrogen envelope of the extremely hot giant exoplanet KELT-9b

Other links:

1. Excited hydrogen detected in exciting gas giant

(by David Ehrenreich, Nature Astronomy News & Views )

2. Detecting the boiling atmosphere of the hottest known exoplanet

(by MPIA press release)

3. The atmosphere of Kelt-9b, the hottest known planet, is being dragged towards its star

(by Carla Alto Observatory press release)

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