CaRM: Exploring the chromatic Rossiter-McLaughlin effect

The cases of HD 189733b and WASP-127b^⋆

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 European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile
⁴ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
⁵ Universidad de La Laguna (ULL), Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁶ Centro de Astrobiología (CSIC-INTA), Crta. Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁷ INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy
⁸ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁹ Département d’Astronomie de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁰ INAF – Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
¹¹ Consejo Superior de Investigaciones Científicas, Spain
¹² Department of Physics, and Institute for Research on Exoplanets, Université de Montréal, Montréal H3T 1J4, Canada
¹³ Physics Institute, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
¹⁴ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
¹⁵ Faculdade de Ciências da Universidade de Lisboa (Departamento de Física), Edifício C8, 1749-016 Lisboa, Portugal
¹⁶ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching b. München, Germany
¹⁷ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁸ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
¹⁹ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy

Received: 1 October 2021
Accepted: 4 January 2022

Abstract

Aims. In this paper we introduce CaRM, a semi-automatic code for the retrieval of broadband transmission spectra of transiting planets through the chromatic Rossiter-McLaughlin method. We applied it to HARPS and ESPRESSO observations of two exoplanets to retrieve the transmission spectrum and we analyze its fitting transmission models.

Methods. We used the strong radius dependence of the Rossiter-McLaughlin (RM) effect amplitude, caused by planetary companions, to measure the apparent radius change caused by the exoplanet atmosphere. In order to retrieve the transmission spectrum, the radial velocities, which were computed over wavelength bins that encompass several spectral orders, were used to simultaneously fit the Keplerian motion and the RM effect. From this, the radius ratio was computed as a function of the wavelength, which allows one to retrieve the low-resolution broadband transmission spectrum of a given exoplanet. CaRM offers the possibility to use two Rossiter-McLaughlin models taken from ARoME and PyAstronomy, associated with a Keplerian function to fit radial velocities during transit observations automatically. Furthermore it offers the possibility to use some methods that could, in theory, mitigate the effect of perturbation in the radial velocities during transits.

Results. We applied CaRM to recover the transmission spectrum of HD 189733b and WASP-127b, with HARPS and ESPRESSO data, respectively. Our results for HD 189733b suggest that the blue part of the spectrum is dominated by Rayleigh scattering, which is compatible with former studies. The analysis of WASP-127b shows a flat transmission spectrum.

Conclusions. The CaRM code allows one to retrieve the transmission spectrum of a given exoplanet using minimal user interaction. We demonstrate that it allows one to compute the low-resolution broadband transmission spectra of exoplanets observed using high-resolution spectrographs such as HARPS and ESPRESSO.