Magnetised winds and their influence in the escaping upper atmosphere of HD 209458b

TL;DR

This study uses 3D MHD simulations to analyze how magnetic fields of the star and planet influence Lyman alpha absorption in HD 209458b, revealing the planetary magnetic field strength that best fits observations.

Contribution

It introduces a detailed 3D MHD modeling approach to assess magnetic effects on exoplanet atmospheric escape and Ly alpha absorption.

Findings

Magnetic fields alter the Ly alpha absorption profile shape.

A planetary magnetic field of less than ~1 G best fits observations.

Magnetic effects influence the size of the planetary magnetosphere.

Abstract

Lyman $\alpha$ observations during an exoplanet transit have proved to be very useful to study the interaction between the stellar wind and the planetary atmosphere. They have been extensively used to constrain planetary system parameters that are not directly observed, such as the planetary mass loss rate. In this way, Ly $\alpha$ observations can be a powerful tool to infer the existence of a planetary magnetic field, since it is expected that the latter will affect the escaping planetary material. To explore the effect that magnetic fields have on the Ly $\alpha$ absorption of HD 209458b, we run a set of 3D MHD simulations including dipolar magnetic fields for the planet and the star. We assume values for the surface magnetic field at the poles of the planet in the range of [0-5] G, and from 1 to 5 G at the poles of the star. Our models also include collisional and photo-ionisation, radiative recombination, and an approximation for the radiation pressure. Our results show that the magnetic field of the planet and the star change the shape of the Ly $\alpha$ absorption profile, since it controls the extent of the planetary magnetosphere and the amount of neutral material inside it. The model that best reproduces the absorption observed in HD 209458b (with canonical values for the stellar wind parameters) corresponds to a dipole planetary field of <~ 1 gauss at the poles.