An ESPRESSO view of the HD 189733 system
Authors: Cristo E. , Esparza Borges E. , Santos N. C. , Demangeon O. , Palle E. , Psaridi A. , Bourrier V. , Faria J. P. , Allart R. , Azevedo Silva T. , Borsa F. , Alibert Y. , Figueira P. , González Hernández J. I. , Lendl M. , Lillo-Box J. , Lo Curto G. , Di Marcantonio P. , Martins C. J. A. P. , Nunes N. J. , Pepe F. , Seidel J. V. , Sousa S. G. , Sozzetti A. , Stangret M. , Suárez Mascareño A. , Tabernero H. M. , Zapatero Osorio M. R. 
Published: November 2023

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, VCB ≈ −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.