An ESPRESSO view of the HD 189733 system

Broadband transmission spectrum, differential rotation, and system architecture^★

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
e-mail: eduardo.cristo14@gmail.com
² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
³ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
⁴ Universidad de La Laguna (ULL), Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁵ Centro de Astrobiología (CSIC-INTA), Crta. Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁶ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile
⁷ INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy
⁸ Département d’astronomie de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁹ INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
¹⁰ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
¹¹ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
¹² Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
¹³ INAF – Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
¹⁴ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122, Padova, Italy
¹⁵ Center for Space and Habitability, University of Bern, Gesellsschaftsstr. 6, 3012 Bern, Switzerland
¹⁶ Department of Physics, and Trottier Institute for Research on Exo-planets, Université de Montréal, Montréal H3T 1J4, Canada

Received: 9 March 2023
Accepted: 2 October 2023

Abstract

Context. The development of state-of-the-art spectrographs has ushered in a new era in the detection and characterization of exoplanetary systems. The astrophysical community now has the ability to gain detailed insights into the composition of atmospheres of planets outside our Solar System. In light of these advancements, several new methods have been developed to probe exoplanetary atmospheres using both broadband and narrowband techniques.

Aims. Our objective is to utilize the high-resolution and precision capabilities of the ESPRESSO instrument to detect and measure the broadband transmission spectrum of HD 189733b’s atmosphere. Additionally, we aim to employ an improved Rossiter–McLaughlin (RM) model to derive properties related to the velocity fields of the stellar surface and to constrain the orbital architecture.

Methods. The RM effect, which strongly depends on a planet’s radius, offers a precise means of measurement. To this end, we divided the observation range of ESPRESSO into wavelength bins, enabling the computation of radial velocities as a function of wavelength. By employing a robust model of the RM effect, we first determined the system’s color-independent properties across the entire spectral range of observations. Subsequently, we measured the planet’s radius from the radial velocities obtained within each wavelength bin, allowing us to extract the exoplanet’s transmission spectrum. Additionally, we employed a retrieval algorithm to fit the transmission spectrum and study the atmospheric properties.

Results. Our results demonstrate a high degree of precision in fitting the radial velocities observed during transit using the improved modeling of the RM effect. We tentatively detect the effect of differential rotation, with a confidence level of 93.4% when considering a rotation period within the photometric literature values, and 99.6% for a broader range of rotation periods. For the former, the amplitude of the differential rotation ratio suggests an equatorial rotation period of 11.45 ± 0.09 days and a polar period of 14.9 ± 2. The addition of differential rotation breaks the latitudinal symmetry, enabling us to measure the true spin-orbit angle, ψ ≈ 13.6 ± 6.9°, and the stellar inclination axis angle, i_* ≈ 71.87_{−5.55°+6.91°}. Moreover, we determine a sub-solar amplitude of the convective blueshift velocity, V_CB ≈ −211₋₆₁₊₆₉ m s⁻¹, which falls within the expected range for a K-dwarf host star and is compatible with both runs. Finally, we successfully retrieved the transmission spectrum of HD 189733b from the high-resolution ESPRESSO data. We observe a significant decrease in radius with increasing wavelength, consistent with the phenomenon of super-Rayleigh scattering.