Non-detection of TiO and VO in the atmosphere of WASP-121b using high-resolution spectroscopy

¹ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
e-mail: smerritt01@qub.ac.uk
² School of Physics, Trinity College Dublin, Dublin 2, Ireland
³ Physikalisches Institut, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁴ School of Chemistry, University of New South Wales, 2052 Sydney, Australia
⁵ Department of Physics and Astronomy, University College London, Gower Street, WC1E 6BT London, UK
⁶ Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁷ Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA
⁸ Physics and Astronomy, Stocker Road, University of Exeter, Exeter, EX4 3RF, UK

Received: 23 December 2019
Accepted: 4 February 2020

Abstract

Thermal inversions have long been predicted to exist in the atmospheres of ultra-hot Jupiters. However, the detection of two species thought to be responsible – titanium oxide and vanadium oxide – remains elusive. We present a search for TiO and VO in the atmosphere of the ultra-hot Jupiter WASP-121b (T_eq ≳ 2400 K), an exoplanet with evidence of VO in its atmosphere at low resolution which also exhibits water emission features in its dayside spectrum characteristic of a temperature inversion. We observed its transmission spectrum with the UV-Visual Echelle Spectrograph at the Very Large Telescope and used the cross-correlation method – a powerful tool for the unambiguous identification of the presence of atomic and molecular species – in an effort to detect whether TiO or VO were responsible for the observed temperature inversion. No evidence for the presence of TiO or VO was found at the terminator of WASP-121b. By injecting signals into our data at varying abundance levels, we set rough detection limits of [VO] ≲−7.9 and [TiO] ≲−9.3. However, these detection limits are largely degenerate with scattering properties and the position of the cloud deck. Our results may suggest that neither TiO or VO are the main drivers of the thermal inversion in WASP-121b; however, until a more accurate line list is developed for VO, we cannot conclusively rule out its presence. Future works will consist of a search for other strong optically-absorbing species that may be responsible for the excess absorption in the red-optical.