A strong H-opacity signal in the near-infrared emission spectrum of the ultra-hot Jupiter KELT-9b

We present the analysis of a spectroscopic secondary eclipse of the hottest transiting exoplanet detected to date, KELT-9b, obtained with the Wide Field Camera 3 aboard the Hubble Space Telescope. We complement these data with literature information on stellar pulsations and Spitzer/Infrared Array Camera and Transiting Exoplanet Survey Satellite eclipse depths of this target to obtain a broadband thermal emission spectrum. Our extracted spectrum exhibits a clear turnoff at 1.4 μm. This points to H- bound-free opacities shaping the spectrum. To interpret the spectrum, we perform grid retrievals of self-consistent 1D equilibrium chemistry forward models, varying the composition and energy budget. The model with solar metallicity and C/O ratio provides a poor fit because the H- signal is stronger than expected, requiring an excess of electrons. This pushes our retrievals toward high atmospheric metallicities ([M/H] = 1.98-0.21+0.19) and a C/O ratio that is subsolar by 2.4σ. We question the viability of forming such a high-metallicity planet, and therefore provide other scenarios to increase the electron density in this atmosphere. We also look at an alternative model in which we quench TiO and VO. This fit results in an atmosphere with a slightly subsolar metallicity and subsolar C/O ratio ([M/H] = -0.22-0.13+0.17, log (C/O) = -0.34-0.34+0.19). However, the required TiO abundances are disputed by recent high-resolution measurements of the same planet.

Désert, J. M.University of Amsterdam (författare)
Pino, L.INAF - Osservatorio Astrofisico di Arcetri (författare)
Line, M. R. (författare)
Bean, J. L. (författare)
Khorshid, N.University of Amsterdam (författare)
Schlawin, E.University of Arizona (författare)
Arcangeli, J.University of Amsterdam (författare)
Barat, S.University of Amsterdam (författare)
Hoeijmakers, H. J.Lund University,Lunds universitet,Astronomi - Genomgår omorganisation,Institutionen för astronomi och teoretisk fysik - Genomgår omorganisation,Naturvetenskapliga fakulteten,Astrofysik,Fysiska institutionen,Institutioner vid LTH,Lunds Tekniska Högskola,Lund Observatory - Undergoing reorganization,Department of Astronomy and Theoretical Physics - Undergoing reorganization,Faculty of Science,Astrophysics,Department of Physics,Departments at LTH,Faculty of Engineering, LTH(Swepub:lu)je7310ho (författare)
Komacek, T. D.University of Maryland (författare)
Mansfield, M.University of Arizona (författare)
Parmentier, V.University of Oxford (författare)
Thorngren, D.University of Montreal (författare)
University of AmsterdamINAF - Osservatorio Astrofisico di Arcetri (creator_code:org_t)

Av författaren/redakt...: Jacobs, B.; Désert, J. M.; Pino, L.; Line, M. R.; Bean, J. L.; Khorshid, N.; visa fler...; Schlawin, E.; Arcangeli, J.; Barat, S.; Hoeijmakers, H. ...; Komacek, T. D.; Mansfield, M.; Parmentier, V.; Thorngren, D.; visa färre...

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