Hydrodynamic Escape of Water Vapor Atmospheres near Very Active Stars

C P Johnstone

doi:10.3847/1538-4357/ab6224

Abstract

Abstract When exposed to the high-energy X-ray and ultraviolet radiation of a very active star, water vapor in the upper atmospheres of planets can be photodissociated and rapidly lost to space. In this paper, I study the chemical, thermal, and hydrodynamic processes in the upper atmospheres of terrestrial planets, concentrating on water-vapor-dominated atmospheres orbiting in the habitable zones of active stars. I consider different stellar activity levels and find very high levels of atmospheric escape in all cases, with the outflowing gas being dominated by atomic hydrogen and oxygen in both their neutral and ion forms. In the lower activity cases, I find that the accumulation of O2 and increases in the D/H ratios in the atmospheres due to mass fractionation are possible, but in the higher activity cases, no mass fractionation takes place. Connecting these results to stellar activity evolution tracks for solar-mass stars, I show that huge amounts of water vapor can be lost, and both the losses and the amount of O2 that can be accumulated in the atmosphere depend sensitively on the star’s initial rotation rate. For an Earth-mass planet in the habitable zone of a low-mass M dwarf, my results suggest that the accumulation of atmospheric O2 is unlikely unless water loss can take place after the star’s most active phase.

Highlights

When exposed to the high energy X-ray and ultraviolet radiation of a very active star, water vapor in the upper atmospheres of planets can be photodissociated and rapidly lost to space
I study the chemical, thermal, and hydrodynamic processes in the upper atmospheres of terrestrial planets, concentrating on water vapor dominated atmospheres orbiting in the habitable zones of active stars
Connecting these results to stellar activity evolution tracks for solar mass stars, I show that huge amounts of water vapor can be lost, and both the losses and the amount of O2 that can be accumulated in the atmosphere depend sensitively on the star’s initial rotation rate

Summary

INTRODUCTION

Atoms (ENAs) created in charge exchange reactions between neutral atmospheric particles and the early solar wind could heat the atmosphere significantly and drive escape, though Lichtenegger et al (2016) found that this mechanism might not lead to significant additional escape. Water vapor in a planet’s upper atmosphere is photodissociated by the high-energy radiative spectrum of its host star creates a large number of other chemical species such as OH, O2, and O3, and causes most of the thermosphere to be filled neutral and ionized hydrogen and oxygen atoms which can flow away from the planet hydrodynamically (Kasting & Pollack 1983; Guo 2019). Fig. 3.— Vertical structures of the hydrodynamically outflowing atmospheres for several cases with different input stellar XUV fluxes, showing outflow velocity (upper-panel), temperature (middle-panel), and ionization fraction (lower-panel) as functions of altitude throughout the simulation domain.

RESULTS

EVOLUTION OF LOSSES AND ACCUMULATION OF O2

DISCUSSION