Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS): VII. Detection of sodium on the long-transiting inflated sub-Saturn KELT-11 b

D. Mounzer, C. Lovis, J. V. Seidel, O. Attia, R. Allart, V. Bourrier, D. Ehrenreich, A. Wyttenbach, N. Astudillo-Defru, T. G. Beatty, H. Cegla, K. Heng, B. Lavie, M. Lendl, C. Melo, F. Pepe, J. Pepper, J. E. Rodriguez, D. Ségransan, S. UdryE. Linder, S. Sousa

Research output: Contribution to journalArticlepeer-review

14 Scopus citations


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+2.36a. 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.

Original languageEnglish (US)
Article numberA1
JournalAstronomy and astrophysics
StatePublished - Dec 1 2022


  • Instrumentation: spectrographs
  • Methods: observational
  • Planets and satellites: atmospheres
  • Planets and satellites: individual: KELT-11 b
  • Techniques: spectroscopic

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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