HST/STIS transmission spectrum of the ultra-hot Jupiter WASP-76 b confirms the presence of sodium in its atmosphere

¹ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
e-mail: cessen@phys.au.dk
² Astronomical Observatory, Institute of Theoretical Physics and Astronomy, Vilnius University, Sauletekio av. 3, 10257 Vilnius, Lithuania
³ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK

Received: 22 November 2019
Accepted: 13 March 2020

Abstract

We present an atmospheric transmission spectrum of the ultra-hot Jupiter WASP-76 b by analyzing archival data obtained with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). The dataset spans three transits, two with a wavelength coverage between 2900 and 5700 Å, and the third one between 5250 and 10 300 Å. From the one-dimensional, time dependent spectra we constructed white and chromatic light curves, the latter with typical integration band widths of ~200 Å. We computed the wavelength dependent planet-to-star radii ratios taking into consideration WASP-76’s companion. The resulting transmission spectrum of WASP-76 b is dominated by a spectral slope of increasing opacity towards shorter wavelengths of amplitude of about three scale heights under the assumption of planetary equilibrium temperature. If the slope is caused by Rayleigh scattering, we derive a lower limit to the temperature of ~870 K. Following-up on previous detection of atomic sodium derived from high resolution spectra, we re-analyzed HST data using narrower bands centered around sodium. From an atmospheric retrieval of this transmission spectrum, we report evidence of sodium at 2.9σ significance. In this case, the retrieved temperature at the top of the atmosphere (10⁻⁵ bar) is 2300₋₃₉₂⁺⁴¹² K. We also find marginal evidence for titanium hydride. However, additional high resolution ground-based data are required to confirm this discovery.