Hybrid simulations of stellar wind interaction with close-in extrasolar planets

E P.G Johansson,U Motschmann,T Bagdonat

doi:10.1051/eas/1041038

E P.G Johansson, U Motschmann + Show 1 more

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https://doi.org/10.1051/eas/1041038

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Abstract

A large fraction of exoplanets orbit their host stars on distances much smaller than any planet in the Solar System. This opens up for the possibility of qualitatively new kinds of stellar wind interaction. The understanding of these interactions may be important both for future detection methods and for mass loss estimates. In this work we investigate such close-in stellar wind interaction using primarily hybrid simulations. These describe plasmas by modeling electrons as a fluid and ions as particles. Two scenarios of stellar wind interaction with unmagnetized, Earth-sized, close-in exoplanets in orbit around a Sun-like star are investigated: 1.) interaction with an extremely hydrodynamically expanding atmosphere, and 2.) quasiparallel stellar wind interaction. In the expanding ionosphere study we can see how the bow shock, magnetic draping and ion composition boundary are pushed upstream, increasing the size of the interaction region. In the process it creates a significant wake behind the planet, largely void of electromagnetic fields and dominated only by the expanding ionosphere. An attempt is also made to analytically estimate the standoff distance and compare these estimates with simulations. In the quasiparallel interaction study we observe how generic features of quasiperpendicular interaction are modified. The dayside bow shock surface is replaced by a vaguely defined parallel shock that destroys the strict division between magnetosheath and stellar wind. The stellar wind also penetrates deeper into the ionosphere.

Highlights

1.1 The first exoplanetsIt was for long just assumed that just as the sky is full of stars like our Sun, the same stars should have planets, just the way our own Sun has planets
A handful of exoplanet detections had been reported over the years but all of them had either been retracted or never became widely accepted1 up until in 1992 when the first confirmed discovery of two planet-mass bodies beyond our Solar System was announced by Wolszczan and Frail (1992)
One could argue that in principle, atmospheres with so different outflow velocities as in our three simulation runs must be in very different physical states and not have the same ionospheric profiles

Summary

Introduction

1.1 The first exoplanetsIt was for long just assumed that just as the sky is full of stars like our Sun, the same stars should have planets, just the way our own Sun has planets. A qualitative argument for how this can occur can be obtained by first considering the simple approximation of an atmosphere as a static ideal gas on a “flat” planet with a homogeneous gravitational field. There is no static solution in a spherically symmetric geometry and one is instead led to use a solution with a non-zero radial atmospheric velocity1 This is basically the same argument as to why there has to be a solar wind (Parker 1958). The magnetic field lines follow the stellar wind, moving more or less radially away from the central star. The result is magnetic field lines shaped almost like Archimedean spiral arms which can be perceived as both moving away from the star and rotating with the star, see Fig. 5.1 This spiral picture has to be understood as only a first approximation . It does not assume that the stellar wind speed is constant

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