The space weather around the exoplanet GJ 436b

I. The large-scale stellar magnetic field

¹ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, IRAP/UMR 5277, 14 avenue Edouard Belin, 31400 Toulouse, France
e-mail: stefano.bellotti@irap.omp.eu
² Science Division, Directorate of Science, European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
³ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁴ Department of Physics, College of Science, United Arab Emirates University, PO Box 15551 Al Ain, UAE
e-mail: rim.fares@uaeu.ac.ae
⁵ Laboratoire Univers et Particules de Montpellier, Université de Montpellier, CNRS, 34095 Montpellier, France
⁶ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland

Received: 17 April 2023
Accepted: 23 June 2023

Abstract

Context. The space environment in which planets are embedded mainly depends on the host star and impacts the evolution of the planetary atmosphere. The quiet M dwarf GJ 436 hosts a close-in hot Neptune which is known to feature a comet-like tail of hydrogen atoms that escaped from its atmosphere due to energetic stellar irradiation. Understanding such star-planet interactions is essential to shed more light on planet formation and evolution theories, in particular the scarcity of Neptune-sized planets below a 3 d orbital period, also known as the ‘Neptune desert’.

Aims. We aimed to characterise the stellar environment around GJ 436, which requires accurate knowledge of the stellar magnetic field. The latter is studied efficiently with spectropolarimetry, since it is possible to recover the geometry of the large-scale magnetic field by applying tomographic inversion on time series of circularly polarised spectra.

Methods. We used spectropolarimetric data collected in the optical domain with Narval in 2016 to compute the longitudinal magnetic field, examine its periodic content via Lomb-Scargle periodogram and Gaussian process regression analysis, and finally reconstruct the large-scale field configuration by means of Zeeman-Doppler imaging.

Results. We found an average longitudinal field of −12 G and a stellar rotation period of 46.6 d using a Gaussian process model and 40.1 d using Zeeman-Doppler imaging, which are both consistent with the literature. The Lomb-Scargle analysis did not reveal any significant periodicity. The reconstructed large-scale magnetic field is predominantly poloidal, dipolar, and axisymmetric, with a mean strength of 16 G. This is in agreement with magnetic topologies seen for other stars of a similar spectral type and rotation rate.