Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS)

VII. Detection of sodium on the long-transiting inflated sub-Saturn KELT-11 b^★

¹ Geneva Observatory, University of Geneva, Chemin Pegasi 51b, 1290 Versoix, Switzerland
e-mail: dany.mounzer@unige.ch
² European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
³ Department of Physics and Institute for Research on Exoplanets, Université de Montréal, Montreal, QC, Canada
⁴ Departamento de Matemática y Física Aplicadas, Universidad Católica de la Santísima Concepción, Concepción, Chile
⁵ Department of Astronomy and Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
⁶ Department of Physics, Astronomy & Astrophysics Group, University of Warwick, Coventry CV4 7AL, UK
⁷ Centre for Exoplanets and Habitability, University of Warwick, Coventry, CV4 7AL, UK
⁸ University of Bern, Center for Space and Habitability, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁹ Ludwig Maximilian University, University Observatory Munich, Scheinerstrasse 1, Munich 81679, Germany
¹⁰ Department of Physics, Lehigh University, 16 Memorial Drive East, Bethlehem, PA 18015, USA
¹¹ Center for Data Intensive and Time Domain Astronomy, Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA
¹² Lucerne School of Engineering and Architecture, Technikumstrasse 21, 6048 Horw, Switzerland
¹³ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal

Received: 10 May 2022
Accepted: 16 July 2022

Abstract

Context. High-resolution transmission spectroscopy has allowed for in-depth information on the composition and structure of exoplanetary atmospheres to be garnered in the last few years, especially in the visible and in the near-infrared. Many atomic and molecular species have been detected thanks to data gathered from state-of-the-art spectrographs installed on large ground-based telescopes. Nevertheless, the Earth daily cycle has been limiting observations to exoplanets with the shortest transits.

Aims. The inflated sub-Saturn KELT-11 b has a hot atmosphere and orbits a bright evolved subgiant star, making it a prime choice for atmospheric characterization. The challenge lies in its transit duration – of more than 7 h – which can only be covered partially or without enough out-of-transit baselines when observed from the ground.

Methods. To overcome this constraint, we observed KELT-11 b with the HARPS spectrograph in series of three consecutive nights, each focusing on a different phase of the planetary orbit: before, during, and after the transit. This allowed us to gather plenty of out-of-transit baseline spectra, which was critical to build a spectrum of the unocculted star with sufficient precision. Telluric absorption lines were corrected using the atmospheric transmission code MOLECFIT. Individual high-resolution transmission spectra were merged to obtain a high signal-to-noise transmission spectrum to search for sodium in KELT-11 b’s atmosphere through the ~5900 Å doublet.

Results. Our results highlight the potential for independent observations of a long-transiting planet over consecutive nights. Our study reveals a sodium excess absorption of 0.28 ± 0.05% and 0.50 ± 0.06% in the Na D1 and D2 lines, respectively. This corresponds to 1.44 and 1.69 times the white-light planet radius in the line cores. Wind pattern modeling tends to prefer day-to-night side winds with no vertical winds, which is surprising considering the planet bloatedness. The modeling of the Rossiter-Mclaughlin effect yields a significantly misaligned orbit, with a projected spin-orbit angle of λ = −77.86_−2.26^+2.36∘.

Conclusions. Belonging to the under-studied group of inflated sub-Saturns, the characteristics of KELT-11 b – notably its extreme scale height and long transit – make it an ideal and unique target for next-generation telescopes. Our results as well as recent findings from HST, TESS, and CHEOPS observations could make KELT-11 b a benchmark exoplanet in atmospheric characterization.