The polar orbit of the warm Neptune GJ 436b seen with VLT/ESPRESSO^★

¹ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
e-mail: vincent.bourrier@unige.ch
² Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
³ Instituto de Astrofísica e Ciências do Espaço, CAUP, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁴ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁵ Instituto de Astrofísica de Canarias, Via Lactea, 38200 La Laguna, Tenerife, Spain
⁶ Universidad de La Laguna, Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁷ INAF, Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
⁸ INAF, Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy
⁹ Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
¹⁰ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹¹ European Southern Observatory, Alonso de Cordova, Vitacura, Santiago, Chile
¹² Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Edifício C8, Campo Grande, PT1749-016 Lisbon, Portugal
¹³ INAF Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
¹⁴ Department of Physics, and Institute for Research on Exoplanets, Université de Montréal, H3T 1J4, Montréal Canada
¹⁵ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
¹⁶ Physics Institute of University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁷ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 190001 Santiago, Chile

Received: 1 November 2021
Accepted: 27 January 2022

Abstract

GJ 436b might be the prototype of warm Neptunes that have undergone late migration induced by an outer companion. Precise determination of the orbital architecture of such systems is critical to constraining their dynamical history and evaluating the role of delayed migration in the exoplanet population. To this purpose we analyzed the Rossiter–McLaughlin (RM) signal of GJ 436 b in two transits – recently observed with ESPRESSO – using three different techniques. The high level of precision achieved in radial velocity (RV) measurements allows us to detect the deviation from the Keplerian orbit, despite the slow rotation of the M dwarf host (v sin i_* = 272.0_−34.0^+40.0 m s⁻¹), and to measure the sky-projected obliquity (λ = 102.5_−18.5^+17.2°). The Reloaded RM technique, which allows the stellar RV field along the transit chord to be analyzed, yields λ = 107.5_−19.3^+26.6° and v sin i_* = 292.9_−49.9^+41.9 m s⁻¹. The RM Revolutions technique, which allows us to fit the spectral profiles from all planet-occulted regions together, yields λ = 114.1_−17.8^+22.8° and v sin i_* = 300.5_−57.0^+45.9 m s⁻¹. The consistent results between these three techniques, and with published results from HARPS/HARPS-N data, confirm the polar orbit of GJ 436b and support the hypothesis that its origin lies in Kozai migration. Results from a joint RM Revolutions analysis of the ESPRESSO, HARPS, and HARPS-N datasets (λ = 113.5_−17.3^+23.3°; v sin i_* = 293.5_−52.2^+43.7 m s⁻¹) combined with a revised stellar inclination (i_* = 35.7_−7.6^+5.9° or 144.2_−5.9^+7.6°) lead us to constrain the 3D obliquity Ψ to 103.2_−11.5^+12.8°.