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

TL;DR

This study detects sodium in the atmosphere of the long-transiting exoplanet KELT-11b using transit spectroscopy, revealing atmospheric composition, wind patterns, and orbital misalignment, highlighting its suitability for future detailed atmospheric studies.

Contribution

First detection of sodium in KELT-11b's atmosphere using multi-night transit spectroscopy, demonstrating the potential of long-duration transits for atmospheric characterization.

Findings

Sodium absorption detected at 1.44 and 1.69 times the planet radius in D1 and D2 lines.

Wind modeling suggests day-to-night side winds without vertical winds.

KELT-11b has a significantly misaligned orbit with a projected spin-orbit angle of about -78 degrees.

Abstract

KELT-11b is an inflated sub-Saturn with a hot atmosphere and that orbits a bright evolved subgiant star, making it a prime choice for atmospheric characterization, but that transits its host star for more than seven hours. We observed this system in series of three consecutive nights with the HARPS spectrograph and report on the analysis of the transmission spectrum obtained from this dataset. Our results highlight the potential for independent observations of a long-transiting planet over consecutive nights. Our study reveals a sodium excess absorption of $0.28 \pm 0.05 %$ and $0.50 \pm 0.06 %$ 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 ${\lambda} = -77.86^{+2.36}_{-2.26}{}^\circ$. 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.