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

Abstract

Giant exoplanets orbiting close to their host stars have high temperatures because of the immense stellar irradiation which they receive. The extreme energy input leads to the expansion of the atmosphere and the escape of neutral hydrogen 1 2 3 . A particularly intriguing case among the hot giant planets is KELT-9b -an exoplanet orbiting very close to an early A-type star with the highest temperature (∼ 4600 K at day-side) among all the exoplanets known so far 4 . The atmospheric composition and dynamic of such a unique planet have been unknown. Here we report the first detection of an extended hot hydrogen atmosphere around KELT-9b. The detection was achieved by measuring the atomic hydrogen absorption during transit with the Balmer H α line, which is unusually strong mainly due to the high level of extreme-ultraviolet radiation from the star. We detected a significant wavelength shift of the H α absorption which is mostly attributed to the planetary orbital motion 5 . The obtained transmission spectrum has a significant line contrast (1.15% extra absorption at the H α line centre). The observation implies that the effective radius at the H α line centre is ∼ 1.64 times the size of the planetary radius, indicating the planet has a largely extended hydrogen envelope close to the size of the Roche lobe ( 1.91 +0.22 −0.26 R planet ) and is probably undergoing dramatic atmosphere escape.We observed two transits of KELT-9b on August 6 and September 21, 2017 with the CARMENES instrument 6 . CARMENES is a high-resolution (R ∼ 94 600 in the visual channel) spectrograph installed at the 3.5 m telescope of the Calar Alto Observatory. Each of the observations lasted for ∼ 6 hours covering the 4 hours transit and 1 hour before and after transit.For each transit dataset, we firstly removed the telluric H 2 O lines at the H α wavelength range and then aligned all the spectra to the stellar rest frame. A reference spectrum was obtained by combining all the spectra observed during out-of-transit. This reference spectrum is regarded as the spectrum of the star and each observed spectrum was then divided by the reference spectrum to remove the stellar H α spectrum. In this way, we obtained planetary absorption spectra with telluric and stellar lines removed. We then binned the planetary absorption spectra from both nights with a 0.01 orbital phase step, i. e. the spectra within each 0.01 phase bin were averaged with the spectral 1 arXiv:1807.00869v1 [astro-ph.EP] 2 Jul 2018 signal-to-noise ratio (SNR) as weight.The obtained planetary absorption spectra are displayed in Fig.1a. The figure shows a black shadow during transit which is the result of the H α absorption. The absorption feature has a significant wavelength shift and this radial velocity (RV) change is mostly attributed to the planetary orbital motion. We modeled the planetary absorption feature assuming it has a Gaussian profile. The semi-amplitude of the planetary orbital motion(K planet ) was set as a free parameter in the model. We included an atmosphere wind speed (...

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