Hydrodynamic escape of water vapor atmospheres near very active stars

TL;DR

This study models how water vapor atmospheres on planets near active stars undergo rapid hydrodynamic escape, leading to significant water loss and potential oxygen buildup, depending on stellar activity levels and evolution.

Contribution

It provides a detailed analysis of atmospheric escape processes under various stellar activity conditions, highlighting the impact of stellar evolution on planetary water retention and oxygen accumulation.

Findings

High atmospheric escape rates dominated by hydrogen and oxygen.

Oxygen accumulation in atmospheres is unlikely around highly active stars.

Water loss can be substantial, especially during early stellar activity phases.

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 O$_2$ 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 O$_2$ 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 O$_2$ is unlikely unless water loss can take place after the star's most active phase.