The Hubble PanCET program: The near-ultraviolet transmission spectrum of WASP-79b

¹ LATMOS, CNRS, Sorbonne Université UVSQ, 11 boulevard d’Alembert, 78280 Guyancourt, France
e-mail: amelie.gressier@latmos.ipsl.fr
² Institut d’Astrophysique de Paris, CNRS, UMR 7095, Sorbonne Université, 98 bis bd Arago, 75014 Paris, France
³ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
⁴ Department of Earth & Planetary Sciences, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
⁵ Department of Physics & Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
⁶ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
⁷ Carnegie Earth & Planets Laboratory, 5241 Broad Branch Road NW, Washington, DC 20015, USA
⁸ School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
⁹ Astronomical Observatory, University of Geneva, Chemin Pegasi 51b, 1290 Versoix, Switzerland
¹⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
e-mail: agressier@stsci.edu
¹¹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191, Gif-sur-Yvette, France
¹² Department of Physics, Utah Valley University, 800 West University Parkway, Orem, UT 8405, USA
¹³ JHU Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
¹⁴ Center of Excellence in Information Systems, Tennessee State University, 3500 John A. Merritt Boulevard, Nashville, TN 37209, USA
¹⁵ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany

Received: 6 July 2022
Accepted: 31 January 2023

Abstract

We present Hubble Space Telescope (HST) transit observations of the Hot-Jupiter WASP-79 b acquired with the Space Telescope Imaging Spectrograph (STIS) in the near ultraviolet (NUV). Two transit observations, part of the PanCET program, are used to obtain the transmission spectra of the planet between 2280 and 3070 Å. We correct for systematic effects in the raw data using the jitter engineering parameters and polynomial modelling to fit the white light curves of the two transits. We observe an increase in the planet-to-star radius ratio at short wavelengths, but no spectrally resolved absorption lines. The difference between the radius ratios at 2400 Å and 3000 Å reaches 0.0191 ± 0.0042 (~4.5−σ). Although the NUV transmission spectrum does not show evidence of hydrodynamical escape, the strong atmospheric features are likely due to species at very high altitudes. We performed a 1D simulation of the temperature and composition of WASP-79 b using Exo-REM. The temperature pressure profile crosses condensation curves of radiatively active clouds, particularly MnS, Mg₂SiO₄, Fe, and Al₂O₃. Still, none of these species produces the level of observed absorption at short wavelengths and can explain the observed increase in the planet’s radius. WASP-79 b’s transit depth reaches 23 scale height, making it one of the largest spectral features observed in an exoplanet at this temperature (~1700 K). The comparison of WASP-79 b’s transmission spectrum with three warmer hot Jupiters shows a similar level of absorption to WASP-178 b and WASP-121 b between 0.2 and 0.3 µm, while HAT-P-41 b’s spectrum is flat. The features could be explained by SiO absorption.