A spectral survey of an ultra-hot Jupiter

Detection of metals in the transmission spectrum of KELT-9 b^★

¹ Observatoire de Genève, University of Geneva, Chemin des Maillettes, 1290 Sauverny, Switzerland
e-mail: jens.hoeijmakers@unige.ch
² Center for Space and Habitability, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
e-mail: jens.hoeijmakers@space.unibe.ch
³ Leiden Observatory, Universiteit Leiden, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
⁴ Anton Pannekoek Institute of Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁵ Cavendish Astrophysics Battcock Centre for Experimental Astrophysics, Cavendish Laboratory, Cambridge University, JJ Thomson Avenue, CB3 0HE Cambridge, UK
⁶ MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge Biomedical Campus, Cambridge University, CB2 0QH Cambridge, UK
⁷ Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, Hardenbergstrasse 36, 10623 Berlin, Germany
⁸ Department of Chemistry and Environmental Science, Medgar Evers College, City University of New York, 1650 Bedford Avenue, Brooklyn, NY 11235, USA

Received: 18 January 2019
Accepted: 3 May 2019

Abstract

Context. KELT-9 b exemplifies a newly emerging class of short-period gaseous exoplanets that tend to orbit hot, early type stars – termed ultra-hot Jupiters. The severe stellar irradiation heats their atmospheres to temperatures of ~4000 K, similar to temperatures of photospheres of dwarf stars. Due to the absence of aerosols and complex molecular chemistry at such temperatures, these planets offer the potential of detailed chemical characterization through transit and day-side spectroscopy. Detailed studies of their chemical inventories may provide crucial constraints on their formation process(es) and evolution history.

Aims. We aim to search the optical transmission spectrum of KELT-9 b for absorption lines by metals using the cross-correlation technique.

Methods. We analysed two transit observations obtained with the HARPS-N spectrograph. We used an isothermal equilibrium chemistry model to predict the transmission spectrum for each of the neutral and singly ionized atoms with atomic numbers between three and 78. Of these, we identified the elements that are expected to have spectral lines in the visible wavelength range and used those as cross-correlation templates.

Results. We detect (>5σ) absorption by Na I, Cr II, Sc II and Y II, and confirm previous detections of Mg I, Fe I, Fe II, and Ti II. In addition, we find evidence of Ca I, Cr I, Co I, and Sr II that will require further observations to verify. The detected absorption lines are significantly deeper than predicted by our model, suggesting that the material is transported to higher altitudes where the density is enhanced compared to a hydrostatic profile, and that the material is part of an extended or outflowing envelope. There appears to be no significant blue-shift of the absorption spectrum due to a net day-to-night side wind. In particular, the strong Fe II feature is shifted by 0.18 ± 0.27 km s⁻¹, consistent with zero. Using the orbital velocity of the planet we derive revised masses and radii of the star and the planet: M_* = 1.978 ± 0.023 M_⊙, R_* = 2.178 ± 0.011 R_⊙, m_p = 2.44 ± 0.70 M_J and R_p = 1.783 ± 0.009 R_J.